mmap vs. O_DIRECT

From: Bill Davidsen
Date: Wed Nov 10 2004 - 15:23:51 EST

I have an application which does a lot of mmap to process its data. The huge waitio time makes me think that mmap isn't doing direct i/o even when things are alligned. Before I start poking the code, is there a reason why direct is not default for i/o in page-size transfers on page size file offsets? I don't have source code, but the parameters of the mmap all seem to satisfy the allignment requirements.

I realize there may be a reason for forcing the i/o through kernel buffers, or for not taking advantage of doing direct i/o whenever possible, it just doesn't jump out at me.

-bill davidsen (davidsen@xxxxxxx)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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