Re: mmap vs. O_DIRECT

Bill Davidsen
2004-11-11T14:47:11Z

Robert Love wrote:
> 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.
>
Sorry, I have to totally disagree, based on a year's experience with 30+
usenet servers which can be run with or without direct. Without direct
the data for every access is copied through the system buffers before
reaching the user program. By using O_DIRECT the waitio time reported
dropped (400-500 users/server) from 40+% to about 14%.

Since the same volume of data and the same number of i/o are being done,
I can't see how doing an extra copy could possibly do anything good!

> 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.

Having seen the results in actual experience using seek/read access, I
am interested in getting the same benefits from the application using
mmap, preferably without rewriting the application to use direct access
explicitly.

I miss your point about synchronous, with hundreds of clients doing
small reads against a 10TB database, the benefit of pushing them through
the page cache isn't obvious. No particular data are in memory long
enough to have much chance of being shared, so it looks like overhead to
me. Feel free to educate me.

I certainly DO want to put more users per server, and direct I/O has
proven itself in actual use. I'm not sure why you think the double copy
is a good thing, but I have good rea$on to want more users per server.

Alan: point on MAP_SHARED taken.

--
bill davidsen <davidsen@tmr.com>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979