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.
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