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: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Andres Freund <andres@anarazel.de>, Amit Kapila <amit.kapila16@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2015-09-08T18:30:19Z
Lists: pgsql-hackers 
On Mon, Sep 7, 2015 at 9:34 AM, Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> Andres Freund wrote:
>> The buffer replacement algorithm for clog is rather stupid - I do wonder
>> where the cutoff is that it hurts.
>>
>> Could you perhaps try to create a testcase where xids are accessed that
>> are so far apart on average that they're unlikely to be in memory? And
>> then test that across a number of client counts?
>>
>> There's two reasons that I'd like to see that: First I'd like to avoid
>> regression, second I'd like to avoid having to bump the maximum number
>> of buffers by small buffers after every hardware generation...
>
> I wonder if it would make sense to explore an idea that has been floated
> for years now -- to have pg_clog pages be allocated as part of shared
> buffers rather than have their own separate pool.  That way, no separate
> hardcoded allocation limit is needed.  It's probably pretty tricky to
> implement, though :-(

Yeah, I looked at that once and threw my hands up in despair pretty
quickly.  I also considered another idea that looked simpler: instead
of giving every SLRU its own pool of pages, have one pool of pages for
all of them, separate from shared buffers but common to all SLRUs.
That looked easier, but still not easy.

I've also considered trying to replace the entire SLRU system with new
code and throwing away what exists today.  The locking mode is just
really strange compared to what we do elsewhere.  That, too, does not
look all that easy.  :-(

-- 
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.