Horton

Re: Speed up Clog Access by increasing CLOG buffers

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Amit Kapila <amit.kapila16@gmail.com>, David Steele <david@pgmasters.net>, Thom Brown <thom@linux.com>, Simon Riggs <simon@2ndquadrant.com>, Jesper Pedersen <jesper.pedersen@redhat.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2016-03-22T14:40:28Z
Lists: pgsql-hackers 
On Tue, Mar 22, 2016 at 6:52 AM, Andres Freund <andres@anarazel.de> wrote:
> I'm actually rather unconvinced that it's all that common that all
> subtransactions are on one page. If you have concurrency - otherwise
> there'd be not much point in this patch - they'll usually be heavily
> interleaved, no?  You can argue that you don't care about subxacts,
> because they're more often used in less concurrent scenarios, but if
> that's the argument, it should actually be made.

But a single clog page holds a lot of transactions - I think it's
~32k.  If you have 100 backends running, and each one allocates an XID
in turn, and then each allocates a sub-XID in turn, and then they all
commit, and then you repeat this pattern, >99% of transactions will be
on a single CLOG page.  And that is a pretty pathological case.

It's true that if you have many short-running transactions interleaved
with occasional long-running transactions, and the latter use
subxacts, the optimization might fail to apply to the long-running
subxacts fairly often.  But who cares?  Those are, by definition, a
small percentage of the overall transaction stream.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Use group updates when setting transaction status in clog.

  2. Improve 64bit atomics support.

  3. Add ProcArrayGroupUpdate wait event.

  4. Make the different Unix-y semaphore implementations ABI-compatible.

  5. Fix broken ALTER INDEX documentation

  6. Code and docs review for commit 3187d6de0e5a9e805b27c48437897e8c39071d45.

  7. Partition the freelist for shared dynahash tables.

  8. Correct StartupSUBTRANS for page wraparound

  9. Make idle backends exit if the postmaster dies.

  10. contrib/sslinfo: add ssl_extension_info SRF

  11. Reduce ProcArrayLock contention by removing backends in batches.

  12. Fix `make installcheck` for serializable transactions.

  13. Lockless StrategyGetBuffer clock sweep hot path.

  14. Reduce sinval synchronization overhead.