Messages in this thread | | | Subject | Re: mmap vs. O_DIRECT | From | Robert Love <> | Date | Wed, 10 Nov 2004 16:13:50 -0500 |
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On Tue, 2004-11-09 at 19:05 -0500, Bill Davidsen wrote: > 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.
Direct I/O (O_DIRECT) will almost assuredly increase I/O wait and degrade I/O performance, not improve it.
I don't think direct I/O is what you want and I am sure that we don't want aligned mmaps to not go through the page cache and be synchronous.
Robert Love
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