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Reply-To: hackers@hub.org, Dan McGuirk <mcguirk@indirect.com> To: hackers@hub.org Subject: [HACKERS] tmin writeback optimization I was doing some profiling of the backend, and noticed that during a certain benchmark I was running somewhere between 30% and 75% of the backend's CPU time was being spent in calls to TransactionIdDidCommit() from HeapTupleSatisfiesNow() or HeapTupleSatisfiesItself() to determine that changed rows' transactions had in fact been committed even though the rows' tmin values had not yet been set. When a query looks at a given row, it needs to figure out whether the transaction that changed the row has been committed and hence it should pay attention to the row, or whether on the other hand the transaction is still in progress or has been aborted and hence the row should be ignored. If a tmin value is set, it is known definitively that the row's transaction has been committed. However, if tmin is not set, the transaction referred to in xmin must be looked up in pg_log, and this is what the backend was spending a lot of time doing during my benchmark. So, implementing a method suggested by Vadim, I created the following patch that, the first time a query finds a committed row whose tmin value is not set, sets it, and marks the buffer where the row is stored as dirty. (It works for tmax, too.) This doesn't result in the boost in real time performance I was hoping for, however it does decrease backend CPU usage by up to two-thirds in certain situations, so it could be rather beneficial in high-concurrency settings. |
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