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

Chuck Lever (cel@monkey.org)
Thu, 15 Jul 1999 11:41:02 -0400 (EDT)

On Thu, 15 Jul 1999, Stephen C. Tweedie wrote:
> >> In conjunction with the disk track buffer, this has the effect of
> >> readahead if you are accessing a mmaped file sequentially in memory
>
> > 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.
>
> Exactly, but for sequential access you are typically measuring
> bandwidth, not latency, and the disk cache hides the latency enough to
> let us keep the disk streaming at media bandwidth. There is definitely
> a latency hit which we could avoid if we pre-submitted the next cluster
> read.

on a related topic,

i notice that try_to_read_ahead (used by both mmap and normal file reads)
will add a page to the page cache and read it in if it isn't already in
the cache. but it won't update a page if the page is already in the
cache, but is not up to date.

does anyone know why ? as an experiment i've changed it to update pages
that aren't up to date, and noticed a performance improvement for mmaps,
but a performance degradation for file reads.

- Chuck Lever

--
corporate:	<chuckl@netscape.com>
personal:	<chucklever@netscape.net> or <cel@monkey.org>


The Linux Scalability project:
	http://www.citi.umich.edu/projects/linux-scalability/




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