Horton

Re: Speed up Clog Access by increasing CLOG buffers

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Michael Paquier <michael.paquier@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Jim Nasby <Jim.Nasby@bluetreble.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2017-03-10T05:21:00Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Mar 9, 2017 at 9:17 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Buildfarm thinks eight wasn't enough.
>> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=clam&dt=2017-03-10%2002%3A00%3A01

> At first I was confused how you knew that this was the fault of this
> patch, but this seems like a pretty indicator:
> TRAP: FailedAssertion("!(curval == 0 || (curval == 0x03 && status !=
> 0x00) || curval == status)", File: "clog.c", Line: 574)

Yeah, that's what led me to blame the clog-group-update patch.

> I'm not sure whether it's related to this problem or not, but now that
> I look at it, this (preexisting) comment looks like entirely wishful
> thinking:
>      * If we update more than one xid on this page while it is being written
>      * out, we might find that some of the bits go to disk and others don't.
>      * If we are updating commits on the page with the top-level xid that
>      * could break atomicity, so we subcommit the subxids first before we mark
>      * the top-level commit.

Maybe, but that comment dates to 2008 according to git, and clam has
been, er, happy as a clam up to now.  My money is on a newly-introduced
memory-access-ordering bug.

Also, I see clam reported in green just now, so it's not 100%
reproducible :-(

			regards, tom lane

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.