Messages in this thread | | | Subject | Re: mmap vs. O_DIRECT | From | Robert Love <> | Date | Thu, 11 Nov 2004 10:41:56 -0500 |
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On Thu, 2004-11-11 at 09:50 -0500, Bill Davidsen wrote:
> 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.
There is a difference between being synchronous and not going through the page cache, although in Linux we don't really have the distinction.
> 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.
BTW, Alan's point on MAP_SHARED is just that you can have the mmap region and the page cached region be one and the same. You still aren't doing direct I/O.
Maybe that is ultimately what you want.
It is rare to see direct I/O perform better when you use it as normal file I/O (e.g. don't perform your own caching and scheduling) but if you really do measure improvements, and if you never reaccess the data (and thus the lack of cache is not a problem), then by all means use it.
But we still don't want to make normal mmap's be direct.
Robert Love
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