I wrote a little app which simulates a text editor writing out
its buffer. Just:
struct line {
char *data;
int length;
struct line *next;
};
walk this linked list, writing the lines out. The input was
`cat linux/kernel/*.c > inputfile' and the output was written
1000 times (300 megs). Benched four different ways of writing the
output:
2.5.34 2.5.34-mm2 2.5.34-mm2-taka
write 54s 54s 55s
fwrite 12.8s 12.8s 12.7s
fwrite_unlocked 11.6s 11.6s 11.5s
writev 39s 33.4s 15.8s
So Janet's patch made a 15% improvement with this test. Yours
dropped it 50% again.
> Yeah, I realized syslogd is using writev against logfiles which are
> opened with O_SYNC flag! I think heavy loaded mail-servers or
> web-servers may get good performance with the new writev
> as they are logging too much.
O_SYNC writev? Ooh, oww, that hurts...
With 2.5.34, writing the 300k file once (1000x less data than above)
with 1024-vector writev's, opened O_SYNC: 68 seconds.
With 2.5.34-mm2-taka the same write takes 0.23 seconds. (I had to write
100x as much data just to get a measurement).
A 300x speedup is nice, but based on these numbers syslogd should be
using fwrite_unlocked() and fflush().
O_SYNC should be eradicated. It's basically always the wrong thing
to do. Applications should write as much stuff as they can and then
run fsync.
>
> It sounds nice.
> I'll rewrite it soon.
>
Great. The test app is at http://www.zip.com.au/~akpm/writev-speed.c
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