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More precisely, what we do here is make the SLRU cache sizes
configurable with new GUCs, so that sites with high concurrency and big
ranges of transactions in flight (resp. multixacts/subtransactions) can
benefit from bigger caches. In order for this to work with good
performance, two additional changes are made:
1. the cache is divided in "banks" (to borrow terminology from CPU
caches), and algorithms such as eviction buffer search only affect
one specific bank. This forestalls the problem that linear searching
for a specific buffer across the whole cache takes too long: we only
have to search the specific bank, whose size is small. This work is
authored by Andrey Borodin.
2. Change the locking regime for the SLRU banks, so that each bank uses
a separate LWLock. This allows for increased scalability. This work
is authored by Dilip Kumar. (A part of this was previously committed as
d172b717c6f4.)
Special care is taken so that the algorithms that can potentially
traverse more than one bank release one bank's lock before acquiring the
next. This should happen rarely, but particularly clog.c's group commit
feature needed code adjustment to cope with this. I (Álvaro) also added
lots of comments to make sure the design is sound.
The new GUCs match the names introduced by bcdfa5f2e2 in the
pg_stat_slru view.
The default values for these parameters are similar to the previous
sizes of each SLRU. commit_ts, clog and subtrans accept value 0, which
means to adjust by dividing shared_buffers by 512 (so 2MB for every 1GB
of shared_buffers), with a cap of 8MB. (A new slru.c function
SimpleLruAutotuneBuffers() was added to support this.) The cap was
previously 1MB for clog, so for sites with more than 512MB of shared
memory the total memory used increases, which is likely a good tradeoff.
However, other SLRUs (notably multixact ones) retain smaller sizes and
don't support a configured value of 0. These values based on
shared_buffers may need to be revisited, but that's an easy change.
There was some resistance to adding these new GUCs: it would be better
to adjust to memory pressure automatically somehow, for example by
stealing memory from shared_buffers (where the caches can grow and
shrink naturally). However, doing that seems to be a much larger
project and one which has made virtually no progress in several years,
and because this is such a pain point for so many users, here we take
the pragmatic approach.
Author: Andrey Borodin <x4mmm@yandex-team.ru>
Author: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Amul Sul, Gilles Darold, Anastasia Lubennikova,
Ivan Lazarev, Robert Haas, Thomas Munro, Tomas Vondra,
Yura Sokolov, Васильев Дмитрий (Dmitry Vasiliev).
Discussion: https://postgr.es/m/2BEC2B3F-9B61-4C1D-9FB5-5FAB0F05EF86@yandex-team.ru
Discussion: https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com
63 lines
1.7 KiB
C
63 lines
1.7 KiB
C
/*
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* clog.h
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*
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* PostgreSQL transaction-commit-log manager
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*
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* Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/access/clog.h
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*/
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#ifndef CLOG_H
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#define CLOG_H
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#include "access/xlogreader.h"
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#include "storage/sync.h"
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#include "lib/stringinfo.h"
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/*
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* Possible transaction statuses --- note that all-zeroes is the initial
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* state.
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*
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* A "subcommitted" transaction is a committed subtransaction whose parent
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* hasn't committed or aborted yet.
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*/
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typedef int XidStatus;
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#define TRANSACTION_STATUS_IN_PROGRESS 0x00
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#define TRANSACTION_STATUS_COMMITTED 0x01
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#define TRANSACTION_STATUS_ABORTED 0x02
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#define TRANSACTION_STATUS_SUB_COMMITTED 0x03
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typedef struct xl_clog_truncate
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{
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int64 pageno;
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TransactionId oldestXact;
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Oid oldestXactDb;
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} xl_clog_truncate;
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extern void TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
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TransactionId *subxids, XidStatus status, XLogRecPtr lsn);
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extern XidStatus TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn);
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extern Size CLOGShmemSize(void);
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extern void CLOGShmemInit(void);
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extern void BootStrapCLOG(void);
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extern void StartupCLOG(void);
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extern void TrimCLOG(void);
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extern void CheckPointCLOG(void);
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extern void ExtendCLOG(TransactionId newestXact);
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extern void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid);
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extern int clogsyncfiletag(const FileTag *ftag, char *path);
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/* XLOG stuff */
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#define CLOG_ZEROPAGE 0x00
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#define CLOG_TRUNCATE 0x10
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extern void clog_redo(XLogReaderState *record);
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extern void clog_desc(StringInfo buf, XLogReaderState *record);
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extern const char *clog_identify(uint8 info);
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#endif /* CLOG_H */
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