|From:||Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>|
|To:||Pg Hackers <pgsql-hackers(at)postgresql(dot)org>|
|Subject:||Accounting of zero-filled buffers in EXPLAIN (BUFFERS)|
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In EXPLAIN (BUFFERS), there are two kinds of cache misses that show up
as "reads" when in fact they are not reads at all:
1. Relation extension, which in fact writes a zero-filled block.
2. The RBM_ZERO_* modes, which provoke neither read nor write.
Here's a suggested fix.
I noticed this because I have some patches in development that hit
these cases a bit more and the numbers didn't match my expectation. I
suppose someone might want a separate counter for zero-filled buffers
(they're still buffer interactions and cache misses) but it seems like
it's probably below the kind of thing we're interested in counting
(though in passing, I notice recently that some kernels keep some free
pages pre-zeroed so they can supply them faster, which is curious).
|Next Message||Andres Freund||2018-04-30 03:13:18||Re: Accounting of zero-filled buffers in EXPLAIN (BUFFERS)|
|Previous Message||Craig Ringer||2018-04-30 02:46:00||Re: Postgres, fsync, and OSs (specifically linux)|