Re: Poor read performance when sequential write presents

[William Lee Irwin III <wli@holomorphy.com>]

On Fri, May 24, 2002 at 02:26:48AM -0700, Andrew Morton wrote:
> Oh absolutely.   That's the reason why 2.4 is beating 2.5 at tiobench with
> more than one thread.  2.5 is alternating fairly between threads and 2.4
> is not.  So 2.4 seeks less.

In one sense or another some sort of graceful transition to unfair
behavior could be considered a kind of thrashing control; how meaningful
that is in the context of disk I/O is a question I can't answer directly,
though. Do you have any comments on this potential strategic unfairness?


On Fri, May 24, 2002 at 02:26:48AM -0700, Andrew Morton wrote:
> I've been testing this extensively on 2.5 + multipage BIO I/O and when you
> increase readahead from 32 pages (two BIOs) to 64 pages (4 BIOs), 2.5 goes
> from perfect to horrid - each threads grabs the disk head and performs many,
> many megabytes of read before any other thread gets a share.  Net effect is
> that the tiobench numbers are great, but any operation which involves
> reading disk has 30 or 60 second latencies.
> Interestingly, it seems specific to IDE.  SCSI behaves well.
> I have tons of traces and debug code - I'll bug Jens about this in a week or
> so.

What kinds of phenomena appear to be associated with IDE's latencies?
I recall some comments from prior IDE maintainers on poor interactions
between generic disk I/O layers and IDE drivers, particularly with
respect to small transactions being given to the drivers to perform.
Are these comments still relevant, or is this of a different nature?


Cheers,
Bill