Horton

Re: Speed up Clog Access by increasing CLOG buffers

Robert Haas <robertmhaas@gmail.com>

View thread View on postgr.es
From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
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: 2017-03-10T02:35:50Z
Lists: pgsql-hackers 
On Thu, Mar 9, 2017 at 9:17 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> I think eight is enough.  Committed with some cosmetic changes.
>
> 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)

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.

The problem with that is the word "before".  There are no memory
barriers here, so there's zero guarantee that other processes see the
writes in the order they're performed here.  But it might be a stretch
to suppose that that would cause this symptom.

Maybe we should replace that Assert() with an elog() and dump out the
actual values.

-- 
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.