In a similar effort to 413c18401, separate out the hot and cold paths in
GenerationAlloc() and SlabAlloc() to avoid having to setup the stack frame
for the hot path.
This additionally adjusts how we use the GenerationContext's freeblock.
Freeblock, when set, is now always empty and we only switch to using it
when the current allocation request finds the current block does not have
enough space and the freeblock is large enough to accomodate the
allocation.
This commit also adjusts GenerationFree() so that if we pfree the final
allocation in the current generation block, we now mark that block as
empty and keep it as the current block. Previously we free'd that block
and set the current block to NULL. Doing that meant we needed a special
case in GenerationAlloc to check if GenerationContext.block was NULL.
So this both reduces free/malloc calls and reduces the work done in
GenerationAlloc().
In passing, improve some comments in aset.c
Discussion: https://postgr.es/m/CAApHDvpHVSJqqb4B4OZLixr=CotKq-eKkbwZqvZVo_biYvUvQA@mail.gmail.com
Many modern compilers are able to optimize function calls to functions
where the parameters of the called function match a leading subset of
the calling function's parameters. If there are no instructions in the
calling function after the function is called, then the compiler is free
to avoid any stack frame setup and implement the function call as a
"jmp" rather than a "call". This is called sibling call optimization.
Here we adjust the memory allocation functions in mcxt.c to allow this
optimization. This requires moving some responsibility into the memory
context implementations themselves. It's now the responsibility of the
MemoryContext to check for malloc failures. This is good as it both
allows the sibling call optimization, but also because most small and
medium allocations won't call malloc and just allocate memory to an
existing block. That can't fail, so checking for NULLs in that case
isn't required.
Also, traditionally it's been the responsibility of palloc and the other
allocation functions in mcxt.c to check for invalid allocation size
requests. Here we also move the responsibility of checking that into the
MemoryContext. This isn't to allow the sibling call optimization, but
more because most of our allocators handle large allocations separately
and we can just add the size check when doing large allocations. We no
longer check this for non-large allocations at all.
To make checking the allocation request sizes and ERROR handling easier,
add some helper functions to mcxt.c for the allocators to use.
Author: Andres Freund
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/20210719195950.gavgs6ujzmjfaiig@alap3.anarazel.de
Here we reduce the block size fields in AllocSetContext, GenerationContext
and SlabContext from Size down to uint32. Ever since c6e0fe1f2, blocks
for non-dedicated palloc chunks can no longer be larger than 1GB, so
there's no need to store the various block size fields as 64-bit values.
32 bits are enough to store 2^30.
Here we also further reduce the memory context struct sizes by getting rid
of the 'keeper' field which stores a pointer to the context's keeper
block. All the context types which have this field always allocate the
keeper block in the same allocation as the memory context itself, so the
keeper block always comes right at the end of the context struct. Add
some macros to calculate that address rather than storing it in the
context.
Overall, in AllocSetContext and GenerationContext, this saves 20 bytes on
64-bit builds which for ALLOCSET_SMALL_SIZES can sometimes mean the
difference between having to allocate a 2nd block and storing all the
required allocations on the keeper block alone. Such contexts are used
in relcache to store cache entries for indexes, of which there can be
a large number in a single backend.
Author: Melih Mutlu
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/CAGPVpCSOW3uJ1QJmsMR9_oE3X7fG_z4q0AoU4R_w+2RzvroPFg@mail.gmail.com
d21ded75f changed the way slab.c works but introduced a bug that meant we
could end up with the slab's curBlocklistIndex pointing to the wrong list.
The condition which was checking for this was failing to account for two
things:
1. The curBlocklistIndex could be 0 as we've currently got no non-full
blocks to put chunks on. In this case, the dlist_is_empty() check cannot
be performed as there can be any number of completely full blocks at that
index.
2. The curBlocklistIndex may be greater than the index we just moved the
block onto. Since we need to ensure we fill up fuller blocks first, we
must reset curBlocklistIndex when changing any blocklist element that's
less than the curBlocklistIndex too.
Reported-by: Takamichi Osumi
Discussion: https://postgr.es/m/TYCPR01MB8373329C6329768D7E093D68EDEB9@TYCPR01MB8373.jpnprd01.prod.outlook.com
Slab has traditionally been fairly slow when compared with the AllocSet or
Generation memory allocators. Part of this slowness came from having to
write out an entire block when we allocate a new block in order to
populate the free list indexes within the block's memory. Additional
slowness came from having to move a block onto another dlist each time we
palloc or pfree a chunk from it.
Here we optimize both of those cases and do a little bit extra to improve
the performance of the slab allocator.
Here, instead of writing out the free list indexes when allocating a new
block, we introduce the concept of "unused" chunks. When a block is first
allocated all chunks are unused. These chunks only make it onto the
free list when they are pfree'd. When allocating new chunks on an
existing block, we have the choice of consuming a chunk from the free list
or an unused chunk. When both exist, we opt to use one from the free
list, as these have been used already and the memory of them is more
likely to be cached by the CPU.
Here we also reduce the number of block lists from there being one for
every possible value of free chunks on a block to just having a small
fixed number of block lists. We keep the 0th block list for completely
full blocks and anything else stores blocks for some range of free chunks
with fuller blocks appearing on lower block list array elements. This
reduces how often we must move a block to another list when we allocate or
free chunks, but still allows us to prefer to put new chunks on fuller
blocks and perhaps allow blocks with fewer chunks to be free'd later
once all their remaining chunks have been pfree'd.
Additionally, we now store a list of "emptyblocks", which are blocks that
no longer contain any allocated chunks. We now keep up to 10 of these
around to avoid having to thrash malloc/free when allocation patterns
continually cause blocks to become free of any allocated chunks only to
allocate more chunks again. Now only once we have 10 of these, we free
the block. This does raise the high water mark for the total memory that
a slab context can consume. It does not seem entirely unreasonable that
we might one day want to make this a property of SlabContext rather than a
compile-time constant. Let's wait and see if there is any evidence to
support that this is required before doing it.
Author: Andres Freund, David Rowley
Tested-by: Tomas Vondra, John Naylor
Discussion: https://postgr.es/m/20210717194333.mr5io3zup3kxahfm@alap3.anarazel.de
Because we added StaticAssertStmt() first before StaticAssertDecl(),
some uses as well as the instructions in c.h are now a bit backwards
from the "native" way static assertions are meant to be used in C.
This updates the guidance and moves some static assertions to better
places.
Specifically, since the addition of StaticAssertDecl(), we can put
static assertions at the file level. This moves a number of static
assertions out of function bodies, where they might have been stuck
out of necessity, to perhaps better places at the file level or in
header files.
Also, when the static assertion appears in a position where a
declaration is allowed, then using StaticAssertDecl() is more native
than StaticAssertStmt().
Reviewed-by: John Naylor <john.naylor@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/941a04e7-dd6f-c0e4-8cdf-a33b3338cbda%40enterprisedb.com
Before commit c6e0fe1f2, functions such as AllocSetFree could pretty
safely presume that they were given a valid chunk pointer for their
own type of context, because the indirect call through a memory
context object and method struct would be very unlikely to work
otherwise. But now, if pfree() is mistakenly invoked on a pointer
to garbage, we have three chances in eight of ending up at one of
these functions. That means we need to take extra measures to
verify that we are looking at what we're supposed to be looking at,
especially in debug builds.
Hence, add code to verify that the chunk's back-link to a block header
leads to a memory context object that satisfies the right sort of
IsA() check. This is still a bit weaker than what we did before,
but for the moment assume that an IsA() check is sufficient.
As a compromise between speed and safety, implement these checks
as Asserts when dealing with small chunks but plain test-and-elogs
when dealing with large (external) chunks. The latter case should
not be too performance-critical, but the former case probably is.
In slab.c, all chunks are small; but nonetheless use a plain test
in SlabRealloc, because that is certainly not performance-critical,
indeed we should be suspicious that it's being called in error.
In aset.c, additionally add some assertions that the "value" field
of the chunk header is within the small range allowed for freelist
indexes. Without that, we might find ourselves trying to wipe
most of memory when CLOBBER_FREED_MEMORY is enabled, or scribbling
on a "freelist header" that's far away from the context object.
Eventually, field experience might show us that it's smarter for
these tests to be active always, but for now we'll try to get
away with just having them as assertions.
While at it, also be more uniform about asserting that context
objects passed as parameters are of the type we expect. Some
places missed that altogether, and slab.c was for no very good
reason doing it differently from the other allocators.
Discussion: https://postgr.es/m/3578387.1665244345@sss.pgh.pa.us
Traditionally, in MEMORY_CONTEXT_CHECKING builds, we only ever marked a
sentinel byte just beyond the requested size if there happened to be
enough space on the chunk to do so. For Slab and Generation context
types, we only rounded the size of the chunk up to the next maxalign
boundary, so it was often not that likely that those would ever have space
for the sentinel given that the majority of allocation requests are going
to be for sizes which are maxaligned. For AllocSet, it was a little
different as smaller allocations are rounded up to the next power-of-2
value rather than the next maxalign boundary, so we're a bit more likely
to have space for the sentinel byte, especially when we get away from tiny
sized allocations such as 8 or 16 bytes.
Here we make more of an effort to allow space so that there is enough room
for the sentinel byte in more cases. This makes it more likely that we'll
detect when buggy code accidentally writes beyond the end of any of its
memory allocations.
Each of the 3 MemoryContext types has been changed as follows:
The Slab allocator will now always set a sentinel byte. Both the current
usages of this MemoryContext type happen to use chunk sizes which were on
the maxalign boundary, so these never used sentinel bytes previously.
For the Generation allocator, we now always ensure there's enough space in
the allocation for a sentinel byte.
For AllocSet, this commit makes an adjustment for allocation sizes which
are greater than allocChunkLimit. We now ensure there is always space for
a sentinel byte. We don't alter the sentinel behavior for request sizes
<= allocChunkLimit. Making way for the sentinel byte for power-of-2
request sizes would require doubling up to the next power of 2. Some
analysis done on the request sizes made during installcheck shows that a
fairly large portion of allocation requests are for power-of-2 sizes. The
amount of additional memory for the sentinel there seems prohibitive, so
we do nothing for those here.
Author: David Rowley
Discussion: https://postgr.es/m/3478405.1661824539@sss.pgh.pa.us
c6e0fe1f2 added a new pointer field to SlabBlock to make it 4 bytes larger
on 32-bit machines. Prior to that commit, the size of that struct was a
multiple of 8, which meant that MAXALIGN(sizeof(SlabBlock)) was the same
as sizeof(SlabBlock), however, after c6e0fe1f2, due to the addition of the
new pointer field to store a pointer to the owning context, that was no
longer true on builds with sizeof(void *) == 4.
This problem was highlighted by an Assert failure which was checking that
the pointer given to pfree() was MAXALIGNED. Various 32-bit ARM buildfarm
animals were failing. These have MAXIMUM_ALIGNOF of 8. The only 32-bit
testing I'd managed to do on c6e0fe1f2 had been on x86, which has a
MAXIMUM_ALIGNOF of 4, therefore did not exhibit this issue.
Here we define Slab_BLOCKHDRSZ and copy what is being done in aset.c and
generation.c for doing calculations based on the size of the context's
block type. This means that SlabAlloc() will now always return a
MAXALIGNed pointer.
This also fixes an incorrect sentinel_ok() check in SlabCheck() which was
incorrectly checking the wrong sentinel byte. This must have previously
not caused any issues due to the fullChunkSize never being large enough to
store the sentinel byte.
Diagnosed-by: Tomas Vondra, Tom Lane
Author: Tomas Vondra, David Rowley
Discussion: https://postgr.es/m/CAA4eK1%2B1JyW5TiL%3DyV-3Uq1CrfnTyn0Xrk5uArt31Z%3D8rgPhXQ%40mail.gmail.com
Whenever we palloc a chunk of memory, traditionally, we prefix the
returned pointer with a pointer to the memory context to which the chunk
belongs. This is required so that we're able to easily determine the
owning context when performing operations such as pfree() and repalloc().
For the AllocSet context, prior to this commit we additionally prefixed
the pointer to the owning context with the size of the chunk. This made
the header 16 bytes in size. This 16-byte overhead was required for all
AllocSet allocations regardless of the allocation size.
For the generation context, the problem was worse; in addition to the
pointer to the owning context and chunk size, we also stored a pointer to
the owning block so that we could track the number of freed chunks on a
block.
The slab allocator had a 16-byte chunk header.
The changes being made here reduce the chunk header size down to just 8
bytes for all 3 of our memory context types. For small to medium sized
allocations, this significantly increases the number of chunks that we can
fit on a given block which results in much more efficient use of memory.
Additionally, this commit completely changes the rule that pointers to
palloc'd memory must be directly prefixed by a pointer to the owning
memory context and instead, we now insist that they're directly prefixed
by an 8-byte value where the least significant 3-bits are set to a value
to indicate which type of memory context the pointer belongs to. Using
those 3 bits as an index (known as MemoryContextMethodID) to a new array
which stores the methods for each memory context type, we're now able to
pass the pointer given to functions such as pfree() and repalloc() to the
function specific to that context implementation to allow them to devise
their own methods of finding the memory context which owns the given
allocated chunk of memory.
The reason we're able to reduce the chunk header down to just 8 bytes is
because of the way we make use of the remaining 61 bits of the required
8-byte chunk header. Here we also implement a general-purpose MemoryChunk
struct which makes use of those 61 remaining bits to allow the storage of
a 30-bit value which the MemoryContext is free to use as it pleases, and
also the number of bytes which must be subtracted from the chunk to get a
reference to the block that the chunk is stored on (also 30 bits). The 1
additional remaining bit is to denote if the chunk is an "external" chunk
or not. External here means that the chunk header does not store the
30-bit value or the block offset. The MemoryContext can use these
external chunks at any time, but must use them if any of the two 30-bit
fields are not large enough for the value(s) that need to be stored in
them. When the chunk is marked as external, it is up to the MemoryContext
to devise its own means to determine the block offset.
Using 3-bits for the MemoryContextMethodID does mean we're limiting
ourselves to only having a maximum of 8 different memory context types.
We could reduce the bit space for the 30-bit value a little to make way
for more than 3 bits, but it seems like it might be better to do that only
if we ever need more than 8 context types. This would only be a problem
if some future memory context type which does not use MemoryChunk really
couldn't give up any of the 61 remaining bits in the chunk header.
With this MemoryChunk, each of our 3 memory context types can quickly
obtain a reference to the block any given chunk is located on. AllocSet
is able to find the context to which the chunk is owned, by first
obtaining a reference to the block by subtracting the block offset as is
stored in the 'hdrmask' field and then referencing the block's 'aset'
field. The Generation context uses the same method, but GenerationBlock
did not have a field pointing back to the owning context, so one is added
by this commit.
In aset.c and generation.c, all allocations larger than allocChunkLimit
are stored on dedicated blocks. When there's just a single chunk on a
block like this, it's easy to find the block from the chunk, we just
subtract the size of the block header from the chunk pointer. The size of
these chunks is also known as we store the endptr on the block, so we can
just subtract the pointer to the allocated memory from that. Because we
can easily find the owning block and the size of the chunk for these
dedicated blocks, we just always use external chunks for allocation sizes
larger than allocChunkLimit. For generation.c, this sidesteps the problem
of non-external MemoryChunks being unable to represent chunk sizes >= 1GB.
This is less of a problem for aset.c as we store the free list index in
the MemoryChunk's spare 30-bit field (the value of which will never be
close to using all 30-bits). We can easily reverse engineer the chunk size
from this when needed. Storing this saves AllocSetFree() from having to
make a call to AllocSetFreeIndex() to determine which free list to put the
newly freed chunk on.
For the slab allocator, this commit adds a new restriction that slab
chunks cannot be >= 1GB in size. If there happened to be any users of
slab.c which used chunk sizes this large, they really should be using
AllocSet instead.
Here we also add a restriction that normal non-dedicated blocks cannot be
1GB or larger. It's now not possible to pass a 'maxBlockSize' >= 1GB
during the creation of an AllocSet or Generation context. Allocations can
still be larger than 1GB, it's just these will always be on dedicated
blocks (which do not have the 1GB restriction).
Author: Andres Freund, David Rowley
Discussion: https://postgr.es/m/CAApHDvpjauCRXcgcaL6+e3eqecEHoeRm9D-kcbuvBitgPnW=vw@mail.gmail.com
Commit 3e98c0bafb added pg_backend_memory_contexts view to display
the memory contexts of the backend process. However its target process
is limited to the backend that is accessing to the view. So this is
not so convenient when investigating the local memory bloat of other
backend process. To improve this situation, this commit adds
pg_log_backend_memory_contexts() function that requests to log
the memory contexts of the specified backend process.
This information can be also collected by calling
MemoryContextStats(TopMemoryContext) via a debugger. But
this technique cannot be used in some environments because no debugger
is available there. So, pg_log_backend_memory_contexts() allows us to
see the memory contexts of specified backend more easily.
Only superusers are allowed to request to log the memory contexts
because allowing any users to issue this request at an unbounded rate
would cause lots of log messages and which can lead to denial of service.
On receipt of the request, at the next CHECK_FOR_INTERRUPTS(),
the target backend logs its memory contexts at LOG_SERVER_ONLY level,
so that these memory contexts will appear in the server log but not
be sent to the client. It logs one message per memory context.
Because if it buffers all memory contexts into StringInfo to log them
as one message, which may require the buffer to be enlarged very much
and lead to OOM error since there can be a large number of memory
contexts in a backend.
When a backend process is consuming huge memory, logging all its
memory contexts might overrun available disk space. To prevent this,
now this patch limits the number of child contexts to log per parent
to 100. As with MemoryContextStats(), it supposes that practical cases
where the log gets long will typically be huge numbers of siblings
under the same parent context; while the additional debugging value
from seeing details about individual siblings beyond 100 will not be large.
There was another proposed patch to add the function to return
the memory contexts of specified backend as the result sets,
instead of logging them, in the discussion. However that patch is
not included in this commit because it had several issues to address.
Thanks to Tatsuhito Kasahara, Andres Freund, Tom Lane, Tomas Vondra,
Michael Paquier, Kyotaro Horiguchi and Zhihong Yu for the discussion.
Bump catalog version.
Author: Atsushi Torikoshi
Reviewed-by: Kyotaro Horiguchi, Zhihong Yu, Fujii Masao
Discussion: https://postgr.es/m/0271f440ac77f2a4180e0e56ebd944d1@oss.nttdata.com
Includes some manual cleanup of places that pgindent messed up,
most of which weren't per project style anyway.
Notably, it seems some people didn't absorb the style rules of
commit c9d297751, because there were a bunch of new occurrences
of function calls with a newline just after the left paren, all
with faulty expectations about how the rest of the call would get
indented.
Nobody really uses this stuff, especially not since we created
valgrind-based infrastructure that does the same thing better.
It is thus unsurprising that the generation.c and slab.c versions
were actually broken. Rather than fix 'em, let's just remove 'em.
Alexander Lakhin
Discussion: https://postgr.es/m/8936216c-3492-3f6e-634b-d638fddc5f91@gmail.com
An AllocSet doubles the size of allocated blocks (up to maxBlockSize),
which means that the current block can represent half of the total
allocated space for the memory context. But the free space in the
current block may never have been touched, so don't count the
untouched memory as allocated for the purposes of
MemoryContextMemAllocated().
Discussion: https://postgr.es/m/ec63d70b668818255486a83ffadc3aec492c1f57.camel@j-davis.com
The bitmap used by SlabCheck to cross-check free chunks in a block used
to be allocated for each SlabCheck call, and was never freed. The memory
leak could be fixed by simply adding a pfree call, but it's actually a
bad idea to do any allocations in SlabCheck at all as it assumes the
state of the memory management as a whole is sane.
So instead we allocate the bitmap as part of SlabContext, which means
we don't need to do any allocations in SlabCheck and the bitmap goes
away together with the SlabContext.
Backpatch to 10, where the Slab context was introduced.
Author: Tomas Vondra
Reported-by: Andres Freund
Reviewed-by: Tom Lane
Backpatch-through: 10
Discussion: https://www.postgresql.org/message-id/20200116044119.g45f7pmgz4jmodxj%40alap3.anarazel.de
Adds accounting of memory allocated in a memory context. Compared to
various ad hoc solutions, the main advantage is that the accounting is
transparent and does not require direct control over allocations (this
matters for use cases where the allocations happen in user code, like
for example aggregate states allocated in a transition functions).
To reduce overhead, the accounting happens at the block level (not for
individual chunks) and only the context immediately owning the block is
updated. When inquiring about amount of memory allocated in a context,
we have to recursively walk all children contexts.
This "lazy" accounting works well for cases with relatively small number
of contexts in the relevant subtree and/or with infrequent inquiries.
Author: Jeff Davis
Reivewed-by: Tomas Vondra, Melanie Plageman, Soumyadeep Chakraborty
Discussion: https://www.postgresql.org/message-id/flat/027a129b8525601c6a680d27ce3a7172dab61aab.camel@j-davis.com
Originally, we treated memory context names as potentially variable in
all cases, and therefore always copied them into the context header.
Commit 9fa6f00b1 rethought this a little bit and invented a distinction
between fixed and variable names, skipping the copy step for the former.
But we can make things both simpler and more useful by instead allowing
there to be two parts to a context's identification, a fixed "name" and
an optional, variable "ident". The name supplied in the context create
call is now required to be a compile-time-constant string in all cases,
as it is never copied but just pointed to. The "ident" string, if
wanted, is supplied later. This is needed because typically we want
the ident to be stored inside the context so that it's cleaned up
automatically on context deletion; that means it has to be copied into
the context before we can set the pointer.
The cost of this approach is basically just an additional pointer field
in struct MemoryContextData, which isn't much overhead, and is bought
back entirely in the AllocSet case by not needing a headerSize field
anymore, since we no longer have to cope with variable header length.
In addition, we can simplify the internal interfaces for memory context
creation still further, saving a few cycles there. And it's no longer
true that a custom identifier disqualifies a context from participating
in aset.c's freelist scheme, so possibly there's some win on that end.
All the places that were using non-compile-time-constant context names
are adjusted to put the variable info into the "ident" instead. This
allows more effective identification of those contexts in many cases;
for example, subsidary contexts of relcache entries are now identified
by both type (e.g. "index info") and relname, where before you got only
one or the other. Contexts associated with PL function cache entries
are now identified more fully and uniformly, too.
I also arranged for plancache contexts to use the query source string
as their identifier. This is basically free for CachedPlanSources, as
they contained a copy of that string already. We pay an extra pstrdup
to do it for CachedPlans. That could perhaps be avoided, but it would
make things more fragile (since the CachedPlanSource is sometimes
destroyed first). I suspect future improvements in error reporting will
require CachedPlans to have a copy of that string anyway, so it's not
clear that it's worth moving mountains to avoid it now.
This also changes the APIs for context statistics routines so that the
context-specific routines no longer assume that output goes straight
to stderr, nor do they know all details of the output format. This
is useful immediately to reduce code duplication, and it also allows
for external code to do something with stats output that's different
from printing to stderr.
The reason for pushing this now rather than waiting for v12 is that
it rethinks some of the API changes made by commit 9fa6f00b1. Seems
better for extension authors to endure just one round of API changes
not two.
Discussion: https://postgr.es/m/CAB=Je-FdtmFZ9y9REHD7VsSrnCkiBhsA4mdsLKSPauwXtQBeNA@mail.gmail.com
This patch makes a number of interrelated changes to reduce the overhead
involved in creating/deleting memory contexts. The key ideas are:
* Include the AllocSetContext header of an aset.c context in its first
malloc request, rather than allocating it separately in TopMemoryContext.
This means that we now always create an initial or "keeper" block in an
aset, even if it never receives any allocation requests.
* Create freelists in which we can save and recycle recently-destroyed
asets (this idea is due to Robert Haas).
* In the common case where the name of a context is a constant string,
just store a pointer to it in the context header, rather than copying
the string.
The first change eliminates a palloc/pfree cycle per context, and
also avoids bloat in TopMemoryContext, at the price that creating
a context now involves a malloc/free cycle even if the context never
receives any allocations. That would be a loser for some common
usage patterns, but recycling short-lived contexts via the freelist
eliminates that pain.
Avoiding copying constant strings not only saves strlen() and strcpy()
overhead, but is an essential part of the freelist optimization because
it makes the context header size constant. Currently we make no
attempt to use the freelist for contexts with non-constant names.
(Perhaps someday we'll need to think harder about that, but in current
usage, most contexts with custom names are long-lived anyway.)
The freelist management in this initial commit is pretty simplistic,
and we might want to refine it later --- but in common workloads that
will never matter because the freelists will never get full anyway.
To create a context with a non-constant name, one is now required to
call AllocSetContextCreateExtended and specify the MEMCONTEXT_COPY_NAME
option. AllocSetContextCreate becomes a wrapper macro, and it includes
a test that will complain about non-string-literal context name
parameters on gcc and similar compilers.
An unfortunate side effect of making AllocSetContextCreate a macro is
that one is now *required* to use the size parameter abstraction macros
(ALLOCSET_DEFAULT_SIZES and friends) with it; the pre-9.6 habit of
writing out individual size parameters no longer works unless you
switch to AllocSetContextCreateExtended.
Internally to the memory-context-related modules, the context creation
APIs are simplified, removing the rather baroque original design whereby
a context-type module called mcxt.c which then called back into the
context-type module. That saved a bit of code duplication, but not much,
and it prevented context-type modules from exercising control over the
allocation of context headers.
In passing, I converted the test-and-elog validation of aset size
parameters into Asserts to save a few more cycles. The original thought
was that callers might compute size parameters on the fly, but in practice
nobody does that, so it's useless to expend cycles on checking those
numbers in production builds.
Also, mark the memory context method-pointer structs "const",
just for cleanliness.
Discussion: https://postgr.es/m/2264.1512870796@sss.pgh.pa.us
Add commentary about what we're doing and why. Apply the method used for
padding in GenerationChunk to AllocChunkData, replacing the rather ad-hoc
solution used in commit 7e3aa03b4. Reorder fields in GenerationChunk so
that the padding calculation will work even if sizeof(size_t) is different
from sizeof(void *) --- likely that will never happen, but we don't need
the assumption if we do it like this. Improve static assertions about
alignment.
In passing, fix a couple of oversights in the "large chunk" path in
GenerationAlloc().
Discussion: https://postgr.es/m/E1eHa4J-0006hI-Q8@gemulon.postgresql.org
Change pg_bsd_indent to follow upstream rules for placement of comments
to the right of code, and remove pgindent hack that caused comments
following #endif to not obey the general rule.
Commit e3860ffa4d wasn't actually using
the published version of pg_bsd_indent, but a hacked-up version that
tried to minimize the amount of movement of comments to the right of
code. The situation of interest is where such a comment has to be
moved to the right of its default placement at column 33 because there's
code there. BSD indent has always moved right in units of tab stops
in such cases --- but in the previous incarnation, indent was working
in 8-space tab stops, while now it knows we use 4-space tabs. So the
net result is that in about half the cases, such comments are placed
one tab stop left of before. This is better all around: it leaves
more room on the line for comment text, and it means that in such
cases the comment uniformly starts at the next 4-space tab stop after
the code, rather than sometimes one and sometimes two tabs after.
Also, ensure that comments following #endif are indented the same
as comments following other preprocessor commands such as #else.
That inconsistency turns out to have been self-inflicted damage
from a poorly-thought-through post-indent "fixup" in pgindent.
This patch is much less interesting than the first round of indent
changes, but also bulkier, so I thought it best to separate the effects.
Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us
The new indent version includes numerous fixes thanks to Piotr Stefaniak.
The main changes visible in this commit are:
* Nicer formatting of function-pointer declarations.
* No longer unexpectedly removes spaces in expressions using casts,
sizeof, or offsetof.
* No longer wants to add a space in "struct structname *varname", as
well as some similar cases for const- or volatile-qualified pointers.
* Declarations using PG_USED_FOR_ASSERTS_ONLY are formatted more nicely.
* Fixes bug where comments following declarations were sometimes placed
with no space separating them from the code.
* Fixes some odd decisions for comments following case labels.
* Fixes some cases where comments following code were indented to less
than the expected column 33.
On the less good side, it now tends to put more whitespace around typedef
names that are not listed in typedefs.list. This might encourage us to
put more effort into typedef name collection; it's not really a bug in
indent itself.
There are more changes coming after this round, having to do with comment
indentation and alignment of lines appearing within parentheses. I wanted
to limit the size of the diffs to something that could be reviewed without
one's eyes completely glazing over, so it seemed better to split up the
changes as much as practical.
Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us
During allocation VALGRIND_MAKE_MEM_DEFINED was called with a pointer
as size. That kind of works, but makes valgrind exceedingly slow for
workloads involving the slab allocator.
Secondly there was an access to memory marked as unreachable within
SlabCheck(). Fix that too.
Author: Tomas Vondra
Discussion: https://postgr.es/m/a6543b6d-6015-99b1-63ef-3ed55a76a730@2ndquadrant.com
Compilers that don't realize that elog(ERROR) doesn't return
complained that SlabRealloc() failed to return a value.
While at it, fix the rather muddled header comment for the function.
Per buildfarm.
The new slab allocator needs different per-allocation information than
the classical aset.c. The definition in 58b25e981 wasn't sufficiently
careful on 32 platforms with 8 byte alignment, leading to buildfarm
failures. That's not entirely easy to fix by just adjusting the
definition.
As slab.c doesn't actually need the size part(s) of the common header,
all chunks are equally sized after all, it seems better to instead
reduce the header to the part needed by all allocators, namely which
context an allocation belongs to. That has the advantage of reducing
the overhead of slab allocations, and also allows for more flexibility
in future allocators.
To avoid spreading the logic about accessing a chunk's context around,
centralize it in GetMemoryChunkContext(), which allows to delete a
good number of lines.
A followup commit will revise the mmgr/README portion about
StandardChunkHeader, and more.
Author: Andres Freund
Discussion: https://postgr.es/m/20170228074420.aazv4iw6k562mnxg@alap3.anarazel.de
The default general purpose aset.c style memory context is not a great
choice for allocations that are all going to be evenly sized,
especially when those objects aren't small, and have varying
lifetimes. There tends to be a lot of fragmentation, larger
allocations always directly go to libc rather than have their cost
amortized over several pallocs.
These problems lead to the introduction of ad-hoc slab allocators in
reorderbuffer.c. But it turns out that the simplistic implementation
leads to problems when a lot of objects are allocated and freed, as
aset.c is still the underlying implementation. Especially freeing can
easily run into O(n^2) behavior in aset.c.
While the O(n^2) behavior in aset.c can, and probably will, be
addressed, custom allocators for this behavior are more efficient
both in space and time.
This allocator is for evenly sized allocations, and supports both
cheap allocations and freeing, without fragmenting significantly. It
does so by allocating evenly sized blocks via malloc(), and carves
them into chunks that can be used for allocations. In order to
release blocks to the OS as early as possible, chunks are allocated
from the fullest block that still has free objects, increasing the
likelihood of a block being entirely unused.
A subsequent commit uses this in reorderbuffer.c, but a further
allocator is needed to resolve the performance problems triggering
this work.
There likely are further potentialy uses of this allocator besides
reorderbuffer.c.
There's potential further optimizations of the new slab.c, in
particular the array of freelists could be replaced by a more
intelligent structure - but for now this looks more than good enough.
Author: Tomas Vondra, editorialized by Andres Freund
Reviewed-By: Andres Freund, Petr Jelinek, Robert Haas, Jim Nasby
Discussion: https://postgr.es/m/d15dff83-0b37-28ed-0809-95a5cc7292ad@2ndquadrant.com
Peter Eisentraut
David Rowley
Bruce Momjian
Tom Lane
Fujii Masao
Jeff Davis
Tomas Vondra
Amit Kapila
Michael Paquier
Heikki Linnakangas
Andres Freund