Phillip Susi wrote:
Bill Davidsen wrote:
Quite honestly, the main place I have found O_DIRECT useful is in
keeping programs doing large i/o quantities from blowing the buffers
and making the other applications run like crap. If you application is
running alone, unless you are very short of CPU or memory avoiding the
copy to an o/s buffer will be down in the measurement noise.
I had a news (usenet) server which normally did 120 art/sec (~480
tps), which dropped to about 50 tps when doing large file copies even
at low priority. By using O_DIRECT the impact essentially vanished, at
the cost of the copy running about 10-15% slower. Changing various
programs to use O_DIRECT only helped when really large blocks of data
were involved, and only when i/o clould be done in a way to satisfy
the alignment and size requirements of O_DIRECT.
If you upgrade to a newer kernel you can try other i/o scheduler
options, default cfq or even deadline might be helpful.
I would point out that if you are looking for optimal throughput and
reduced cpu overhead, and avoid blowing out the kernel fs cache, you
need to couple aio with O_DIRECT. By itself O_DIRECT will lower
throughput because there will be brief pauses between each IO while the
application prepares the next buffer. You can overcome this by posting
a few pending buffers concurrently with aio, allowing the kernel to
always have a buffer ready for the next io as soon as the previous one
completes.
A good point, but in this case there was no particular urgency, other
than not to stop the application while doing background data moves. The
best way to do it would have been to put it where it belonged in the
first place :-(
--
bill davidsen <[email protected]>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
