Re: [RFC] - Some notions that I would like comments on
Jeremy Fitzhardinge (jeremy@goop.org)
Mon, 19 Jul 1999 14:18:18 -0700 (PDT)
On 19-Jul-99 Stephen C. Tweedie wrote:
> Hi,
>
> On Sun, 18 Jul 1999 14:37:35 -0400 (EDT), Chuck Lever <cel@monkey.org>
> said:
>
>> right - i should have specified that the "readbehind" has negative effects
>> for random accesses, but are not much of an issue for sequential accesses
>> of an mmap'd file.
>
> No. "Readaround" works well for the random access patterns seen when
> accessing executable code. When running a library function, you
> often end up invoking a lot of related functions which are called by
> that function, and those are as likely to be earlier in the library
> image as later.
>
> Doing the clustered readaround in this situation maximises the speed at
> which we page in contiguous large chunks of the library. Because we are
> hitting nearby functions backwards as well as forwards, the lack of
> universal readahead in this case really doesn't hurt much.
I'm doing some work at the moment to restructure ELF files so that read-ahead
is always the best thing to do. It collects usage statistics on the ELF file,
and regroups it so that the pages appear on disk in the order in which they're
used. This is primarily useful for speeding up the initial flurry of demand
loading when you start up something big (eg, netscape).
J
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