Re: clustering page-ins

Chuck Lever (
Mon, 2 Aug 1999 16:07:23 -0400 (EDT)

On Fri, 30 Jul 1999, Stephen C. Tweedie wrote:
> On Thu, 29 Jul 1999 15:04:25 -0400 (EDT), Chuck Lever <> said:
> > yes, since they are sharing the same file in the same page cache it
> > would be silly to trigger read-ahead needlessly, even if it finds all the
> > pages already in the page cache.
> You have totally lost me. Given that each thread will share both the
> vmas _and_ the page tables, multiple threads accesing the pages is
> absolutely no different to one thread faulting pages in the same order.

i'm not clear on when vma's are shared.

say separate processes are sharing a file (say, a system log file, or
/etc/passwd) and reads are sequential. the problem isn't scheduling too
many read-ahead I/O's, it's that every process reading this "shared" file
sequentially will trigger the read-ahead code. this generates a whole
bunch of page cache lookups that could be avoided. not a big deal, but it
would be nice if the logic was smart enough to avoid it -- if read-ahead
is, say, 4 clusters, that's 64 extra page cache lookups every time that
part of the file is requested by some process.

- Chuck Lever

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