Horton

Re: [PING] [PATCH v2] parallel pg_restore: avoid disk seeks when jumping short distance forward

Dimitrios Apostolou <jimis@gmx.net>

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From: Dimitrios Apostolou <jimis@gmx.net>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@lists.postgresql.org
Date: 2025-06-11T23:25:00Z
Lists: pgsql-hackers 
On Wed, 11 Jun 2025, Nathan Bossart wrote:

> On Wed, Jun 11, 2025 at 12:32:58AM +0200, Dimitrios Apostolou wrote:
>> what read-seek pattern do you see on the system call level (as shown by
>> strace)? In pg_restore it was a constant loop of read(4K)-lseek(8-16K).
>
> For fseeko(), sizes less than 4096 produce a repeating pattern of read()
> calls followed by approximately (4096 / size) lseek() calls.  For greater
> sizes, it's just a stream of lseek().

This is the opposite of what the link you shared before describes, so 
maybe glibc has changed its behaviour to improve performance.

Anyway, the fact that fseek(>4096) produces a stream of lseek()s, means 
that most likely no I/O is happening. You need to issue a getc() after 
each fseeko(), like pg_restore is doing.


Dimitris

Commits

  1. Avoid short seeks in pg_restore.

  2. Don't rely on zlib's gzgetc() macro.

  3. Add more TAP test coverage for pg_dump.

  4. Split 002_pg_dump.pl into two test files.

  5. Align the data block sizes of pg_dump's various compression modes.

  6. Fix serious performance problems in LZ4Stream_read_internal.

  7. Fix poor buffering logic in pg_dump's lz4 and zstd compression code.

  8. Fix issue with reading zero bytes in Gzip_read.

  9. Restore test coverage of LZ4Stream_gets().