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    SubjectRe: [RFC] - Some notions that I would like comments on


    On Wed, 14 Jul 1999, Stephen C. Tweedie wrote:

    > Hi,
    >
    > On Tue, 13 Jul 1999 20:31:48 +0200, Jamie Lokier
    > <lkd@tantalophile.demon.co.uk> said:
    >
    > > This is done already and automatically -- it's called readahead.
    > > The kernel detects when you're reading sequentially and then
    > > reads ahead.
    >
    > > We don't do it on mmaped areas yet though.
    >
    > In 2.2, we do --- sort of. We "readaround", instead --- we page in an
    > entire contiguuous 64k-long, 64k-aligned chunk of the file by default
    > (less on low-memory machines).

    Looks like clusters of pages to me, but should be as good as readaround,
    if readaround means read-behind-and-ahead in one chunk. It is also
    probably simpler to implement.

    > In conjunction with the disk track buffer, this has the effect of
    > readahead if you are accessing a mmaped file sequentially in memory ---
    > on 2.2, programs which use mmap (such as grep) will happily run at disk
    > speed now. The readaround algorithm has the advantage over readahead
    > that it also works very well when randomly paging in executable files
    > (this is why netscape loads 2 to 3 times faster on 2.2 than on 2.0 on my
    > machines).

    I donnot follow you here. I understand that we read 64 K at a time, and
    not that we queue an IO ahead, so we should still have the latency of the
    data going from the disk-cache (hopefully) to memory. But as you know
    modern disks have large cache, perform large prefetching, and buses are
    very fast. In such a situation, queuing IO ahead is not as interesting as
    it used to be in the past.

    By the way, I do think that clustering or read-around was the right thing
    to implement for page IO.

    > --Stephen

    Gérard.


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