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Commit5364b35

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Increase maximum number of clog buffers.
Benchmarking has shown that the current number of clog buffers limitsscalability. We've previously increased the number in33aaa13, butthat's not sufficient with a large number of clients.We've benchmarked the cost of increasing the limit by benchmarking worstcase scenarios; testing showed that 128 buffers don't cause aregression, even in contrived scenarios, whereas 256 doesThere are a number of more complex patches flying around to addressvarious clog scalability problems, but this is simple enough that we canget it into 9.6; and is beneficial even after those patches have beenapplied.It is a bit unsatisfactory to increase this in small steps every fewreleases, but a better solution seems to require a rewrite of slru.c;not something done quickly.Author: Amit Kapila and Andres FreundDiscussion: CAA4eK1+-=18HOrdqtLXqOMwZDbC_15WTyHiFruz7BvVArZPaAw@mail.gmail.com
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  • src/backend/access/transam

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‎src/backend/access/transam/clog.c

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Original file line numberDiff line numberDiff line change
@@ -417,30 +417,23 @@ TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
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/*
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* Number of shared CLOG buffers.
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*
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* Testing during the PostgreSQL 9.2 development cycle revealed that on a
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* large multi-processor system, it was possible to have more CLOG page
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* requests in flight at one time than the number of CLOG buffers which existed
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* at that time, which was hardcoded to 8. Further testing revealed that
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* performance dropped off with more than 32 CLOG buffers, possibly because
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* the linear buffer search algorithm doesn't scale well.
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* On larger multi-processor systems, it is possible to have many CLOG page
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* requests in flight at one time which could lead to disk access for CLOG
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* page if the required page is not found in memory. Testing revealed that we
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* can get the best performance by having 128 CLOG buffers, more than that it
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* doesn't improve performance.
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*
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* Unconditionallyincreasing the number of CLOG buffers to32 did not seem
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*likea good idea, because it would increase the minimum amount of shared
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*memoryrequired to start, which could be a problem for people running very
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*smallconfigurations. The following formula seems to represent a reasonable
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* Unconditionallykeeping the number of CLOG buffers to128 did not seem like
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* a good idea, because it would increase the minimum amount of shared memory
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* required to start, which could be a problem for people running very small
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* configurations. The following formula seems to represent a reasonable
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* compromise: people with very low values for shared_buffers will get fewer
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* CLOG buffers as well, and everyone else will get 32.
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*
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* It is likely that some further work will be needed here in future releases;
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* for example, on a 64-core server, the maximum number of CLOG requests that
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* can be simultaneously in flight will be even larger. But that will
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* apparently require more than just changing the formula, so for now we take
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* the easy way out.
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* CLOG buffers as well, and everyone else will get 128.
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*/
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Size
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CLOGShmemBuffers(void)
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{
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returnMin(32,Max(4,NBuffers /512));
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returnMin(128,Max(4,NBuffers /512));
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}
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/*

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