Re: pglz performance

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Petr Jelinek <petr(at)2ndquadrant(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Andrey Borodin <x4mmm(at)yandex-team(dot)ru>, Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>, Michael Paquier <michael(at)paquier(dot)xyz>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Vladimir Leskov <vladimirlesk(at)yandex-team(dot)ru>
Subject: Re: pglz performance
Date: 2019-08-04 11:52:37
Message-ID: 20190804115237.nliwf6hhaqfd62ps@development
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On Sun, Aug 04, 2019 at 02:41:24AM +0200, Petr Jelinek wrote:
>On 02/08/2019 21:48, Tomas Vondra wrote:
>>On Fri, Aug 02, 2019 at 11:20:03AM -0700, Andres Freund wrote:
>>>>Another question is whether we'd actually want to include the code in
>>>>core directly, or use system libraries (and if some packagers might
>>>>decide to disable that, for whatever reason).
>>>I'd personally say we should have an included version, and a
>>>--with-system-... flag that uses the system one.
>>OK. I'd say to require a system library, but that's a minor detail.
>Same here.
>Just so that we don't idly talk, what do you think about the attached?
>- adds new GUC compression_algorithm with possible values of pglz
>(default) and lz4 (if lz4 is compiled in), requires SIGHUP
>- adds --with-lz4 configure option (default yes, so the configure
>option is actually --without-lz4) that enables the lz4, it's using
>system library
>- uses the compression_algorithm for both TOAST and WAL compression (if on)
>- supports slicing for lz4 as well (pglz was already supported)
>- supports reading old TOAST values
>- adds 1 byte header to the compressed data where we currently store
>the algorithm kind, that leaves us with 254 more to add :) (that's an
>extra overhead compared to the current state)
>- changes the rawsize in TOAST header to 31 bits via bit packing
>- uses the extra bit to differentiate between old and new format
>- supports reading from table which has different rows stored with
>different algorithm (so that the GUC itself can be freely changed)


>Simple docs and a TAP test included.
>I did some basic performance testing (it's not really my thing though,
>so I would appreciate if somebody did more).
>I get about 7x perf improvement on data load with lz4 compared to pglz
>on my dataset but strangely only tiny decompression improvement.
>Perhaps more importantly I also did before patch and after patch tests
>with pglz and the performance difference with my data set was <1%.
>Note that this will just link against lz4, it does not add lz4 into
>PostgreSQL code-base.

WFM, although I think Andres wanted to do both (link against system and
add lz4 code as a fallback). I think the question is what happens when
you run with lz4 for a while, and then switch to binaries without lz4
support. Or when you try to replicate from lz4-enabled instance to an
instance without it. Especially for physical replication, but I suppose
it may affect logical replication with binary protocol?

A very brief review:

1) I wonder what "runstatedir" is about.

2) This seems rather suspicious, and obviously the comment is now
entirely bogus:

/* Check that off_t can represent 2**63 - 1 correctly.
We can't simply define LARGE_OFF_T to be 9223372036854775807,
since some C++ compilers masquerading as C compilers
incorrectly reject 9223372036854775807. */
-#define LARGE_OFF_T (((off_t) 1 << 62) - 1 + ((off_t) 1 << 62))
+#define LARGE_OFF_T ((((off_t) 1 << 31) << 31) - 1 + (((off_t) 1 << 31) << 31))

3) I can't really build without lz4:

config.status: linking src/makefiles/Makefile.linux to src/Makefile.port
pg_lzcompress.c: In function ‘pg_compress_bound’:
pg_lzcompress.c:892:22: error: ‘SIZEOF_PG_COMPRESS_HEADER’ undeclared (first use in this function)
return slen + 4 + SIZEOF_PG_COMPRESS_HEADER;
pg_lzcompress.c:892:22: note: each undeclared identifier is reported only once for each function it appears in

4) I did a simple test with physical replication, with lz4 enabled on
both sides (well, can't build without lz4 anyway, per previous point).
It immediately failed like this:

FATAL: failed to restore block image
CONTEXT: WAL redo at 0/5000A40 for Btree/INSERT_LEAF: off 138
LOG: startup process (PID 15937) exited with exit code 1

This is a simple UPDATE on a trivial table:

create table t (a int primary key);
insert into t select i from generate_series(1,1000) s(i);
update t set a = a - 100000 where random () < 0.1;

with some checkpoints to force FPW (and wal_compression=on, of course).

I haven't tried `make check-world` but I suppose some of the TAP tests
should fail because of this. And if not, we need to improve coverage.

5) I wonder why compression_algorithm is defined as PGC_SIGHUP. Why not
to allow users to set it per session? I suppose we might have a separate
option for WAL compression_algorithm.

6) It seems a bit strange that pg_compress/pg_decompress are now defined
in pglz_compress.{c,h}. Maybe we should invent src/common/compress.c?


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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