Messages in this thread |  | | | Date | Thu, 12 Sep 2002 20:31:16 -0700 | | From | Andrew Morton <> | | Subject | Re: [patch] readv/writev rework |
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Hirokazu Takahashi wrote: > > Hello, > > > > Your readv/writev patch interested me and I checked it. > > > I found we also have a chance to improve normal writev. > > > > > > a_ops->prepare_write() and a_ops->commit_write will have a > > > penalty when I/O size isn't PAGE_SIZE. > > > With following patch generic_file_write_nolock() will try to > > > make each I/O size become PAGE_SIZE. > > > > > > > Certainly makes a lot of sense. If an application has a large > > number of small objects which are to be appended to a file, and > > they are not contiguous in user memory then this patch makes > > writev() a very attractive way of doing that. Tons faster.
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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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