On Thu, Aug 8, 2013 at 9:27 PM, Andres Freund <and...@2ndquadrant.com> wrote: > On 2013-08-08 16:12:06 -0500, Jon Nelson wrote: ...
>> At this point I'm convinced that the issue is a pathological case in >> ext4. The performance impact disappears as soon as the unwritten >> extent(s) are written to with real data. Thus, even though allocating >> files with posix_fallocate is - frequently - orders of magnitude >> quicker than doing it with write(2), the subsequent re-write can be >> more expensive. At least, that's what I'm gathering from the various >> threads. > > >> Why this issue didn't crop up in earlier testing and why I >> can't seem to make test_fallocate do it (even when I modify >> test_fallocate to write to the newly-allocated file in a mostly-random >> fashion) has me baffled. > > It might be kernel version specific and concurrency seems to play a > role. If you reproduce the problem, could you run a "perf record -ga" to > collect a systemwide profile? Finally, an excuse to learn how to use 'perf'! I'll try to provide that info when I am able. > There's some more things to test: > - is the slowdown dependent on the scale? I.e is it visible with -j 1 -c > 1? scale=1 (-j 1 -c 1): with fallocate: 685 tps without: 727 scale=20 with fallocate: 129 without: 402 scale=40 with fallocate: 163 without: 511 > - Does it also occur in synchronous_commit=off configurations? Those > don't fdatasync() from so many backends, that might play a role. With synchronous_commit=off, the performance is vastly improved. Interestingly, the fallocate case is (immaterially) faster than the non-fallocate case: 3766tps vs 3700tps. I tried a few other wal_sync_methods besides the default of fdatasync, all with scale=80. fsync: 198 tps (with fallocate) vs 187. open_sync: 195 tps (with fallocate) vs. 192 > - do bulkloads see it? E.g. the initial pgbench load? time pgbench -s 200 -p 54320 -d pgb -i with fallocate: 2m47s without: 2m50s Hopefully the above is useful. -- Jon -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
