pgsql: Reuse zstd decompression contexts when restoring WAL images

Reuse zstd decompression contexts when restoring WAL images

Restoring each zstd-compressed full-page image currently creates and
destroys a decompression context. One decompression context gets now
saved in a XLogReaderState on first use, reused for the decompression of
any follow-up zstd-compressed pages.

The decompression context is freed in XLogReaderFree(). In non-FRONTEND
builds, a memory context reset callback is registered to make sure that
the decompression context does not leak. Note that zstd has APIs to
either initialize a pre-allocated decompression context or assign custom
allocation and free routines, but none of these are available for
builds that dynamically link to the zstd library (ZSTDLIB_STATIC_API).

Tomas Vondra has done some benchmarking showing that for a FPI-heavy set
of WAL records, reusing the decompression context can nicely improve the
recovery time, bringing it close to lz4. In my own measurements, I have
seen up to a 4% faster recovery time when replaying records made mostly
of zstd-compressed full-page images between HEAD and this improvement.
Andrey has seen better numbers than that.

Author: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://www.postgresql.org/message-id/asRsOl73nf8GeBFd@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ce3387b74b708a679282ba5c72990b532d661bb8

Modified Files
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src/backend/access/transam/xlogreader.c | 51 +++++++++++++++++++++++++++++++--
src/include/access/xlogreader.h | 9 ++++++
2 files changed, 57 insertions(+), 3 deletions(-)