Horton

Re: Speed up Clog Access by increasing CLOG buffers

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Dilip Kumar <dilipbalaut@gmail.com>, Robert Haas <robertmhaas@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2016-10-05T06:35:19Z
Lists: pgsql-hackers 
Hi,

After collecting a lot more results from multiple kernel versions, I can 
confirm that I see a significant improvement with 128 and 192 clients, 
roughly by 30%:

                            64        128        192
     ------------------------------------------------
      master             62482      43181      50985
      granular-locking   61701      59611      47483
      no-content-lock    62650      59819      47895
      group-update       63702      64758      62596

But I only see this with Dilip's workload, and only with pre-4.3.0 
kernels (the results above are from kernel 3.19).

With 4.5.5, results for the same benchmark look like this:

                            64        128        192
     ------------------------------------------------
      master             35693      39822      42151
      granular-locking   35370      39409      41353
      no-content-lock    36201      39848      42407
      group-update       35697      39893      42667

That seems like a fairly bad regression in kernel, although I have not 
identified the feature/commit causing it (and it's also possible the 
issue lies somewhere else, of course).

With regular pgbench, I see no improvement on any kernel version. For 
example on 3.19 the results look like this:

                            64        128        192
     ------------------------------------------------
      master             54661      61014      59484
      granular-locking   55904      62481      60711
      no-content-lock    56182      62442      61234
      group-update       55019      61587      60485

I haven't done much more testing (e.g. with -N to eliminate collisions 
on branches) yet, let's see if it changes anything.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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.