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Re: Setting behavior for clustered IO


From: Samuel Thibault
Subject: Re: Setting behavior for clustered IO
Date: Tue, 3 Apr 2012 00:05:39 +0200
User-agent: Mutt/1.5.21+34 (58baf7c9f32f) (2010-12-30)

Sergio López, le Tue 03 Apr 2012 00:00:02 +0200, a écrit :
> 2012/4/2 Samuel Thibault <address@hidden>:
> > Sergio Lopez, le Mon 02 Apr 2012 17:09:08 +0200, a écrit :
> >> El Mon, 02 Apr 2012 00:23:03 +0300
> >> Maksym Planeta <address@hidden> escribió:
> >> > This function allows user advise the kernel about how to handle paging
> >> > input/output in specified memory range. There are several behaviors,
> >> > like RANDOM, NORMAL, SEQUENTIAL, WILLNEED and DONTNEED. From the page
> >> > fault handler's point of view these behaviors differ only in size of
> >> > memory chunk that will be read ahead.
> >>
> >> I don't think the kernel should be the one to be advised, but the
> >> filesystem translators. These are the ones who really know current and
> >> future (as they control most of the operations) state of the object,
> >
> > Do they really?  We discussed about it with neal a long time ago, and we
> > believed that Mach was at a better position, because it knows about
> > each and every mapping. Say for instance that two processes map the same
> > file, and access it concurrently, but in a different way. AIUI, the
> > translators will get data requests without indication of what mapping is
> > pulling it, and thus no correlation between them, thus seemingly
> > random.

I realize that I was talking for read-ahead, which has not much to do
with cluster size.

Samuel



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