[hackers-jp: 179] Fw: [HACKERS] WAL: O_DIRECT and multipage-writer

[TANIDA Yutaka]

谷田です。めざとく見つけてみる。

wal_sync_method=OPEN_DIRECTは去年似たようなものを私も自分で作ってみて
（そのときは確かopen_syncをつぶした）20%ぐらい早くなったんだけど、実際に
堅牢性がどうなのかは試してないです。たしかperformanceに同じような議論が
あったと思います。

write()のほうは、buffer managerの方にも応用できるのかなぁ。


Forwarded by TANIDA Yutaka <tanida ＠ sra.co.jp>
----------------------- Original Message -----------------------
 From:    ITAGAKI Takahiro <itagaki.takahiro ＠ lab.ntt.co.jp>
 To:      pgsql-hackers ＠ postgresql.org,
          pgsql-patches ＠ postgresql.org
 Date:    Tue, 25 Jan 2005 18:06:23 +0900
 Subject: [HACKERS] WAL: O_DIRECT and multipage-writer
----

Hello, all.

I think that there is room for improvement in WAL. 
Here is a patch for it.
  - Multiple pages are written in one write() if it is contiguous.
  - Add 'open_direct' to wal_sync_method.

WAL writer writes one page in one write(). This is not efficient
when wal_sync_method is 'open_sync', because the writer waits for
IO completions at each write(). Multipage-writer can reduce syscalls
and improve IO throughput. 

'open_direct' uses O_DIRECT instead of O_SYNC. O_DIRECT implies synchronous
writing, so it may show the tendency like open_sync. But maybe it can reduce
memcpy() and save OS's disk cache memory.

I benchmarked this patch with pgbench. It works well and 
improved 50% of tps on my machine. WAL seems to be bottle-neck
on machines with poor disks.

This patch has not yet tested enough. I would like it to be examined much
and taken into PostgreSQL.

There are still many TODOs:
  * Is this logic really correct?
  - O_DIRECT_BUFFER_ALIGN should be adjusted to runtime, not compile time.
  - Consider to use writev() instead of write().
    Buffers are noncontiguous when WAL ring buffer rotates.
  - If wan_sync_method is not open_direct, XLOG_EXTRA_BUFFERS can be 0.


Sincerely,
ITAGAKI Takahiro



-- pgbench result --

$ ./pgbench -s 100 -c 50 -t 400

- 8.0.0 default + fsync:
    tps = 20.630632 (including connections establishing)
    tps = 20.636768 (excluding connections establishing)
- multipage-writer + open_direct:
    tps = 33.761917 (including connections establishing)
    tps = 33.778320 (excluding connections establishing)

Environment:
  OS     : Linux kernel 2.6.9
  CPU    : Pentium 4 3GHz
  disk   : ATA 5400rpm (Data and WAL are placed on same partition.)
  memory : 1GB
  config : shared_buffers=10000, wal_buffers=256,
           XLOG_SEG_SIZE=256MB, checkpoint_segment=4

---
ITAGAKI Takahiro <itagaki.takahiro ＠ lab.ntt.co.jp>
NTT Cyber Space Laboratories
Nippon Telegraph and Telephone Corporation.

--------------------- Original Message Ends --------------------

-- 
TANIDA Yutaka <tanida ＠ sra.co.jp>