On Thu, 11 Jan 2007, Andrew Morton wrote:
> On Thu, 11 Jan 2007 03:04:00 -0800 (PST)
> dean gaudet <[email protected]> wrote:
>
> > On Tue, 9 Jan 2007, Neil Brown wrote:
> >
> > > Imagine a machine with lots of memory - say 100Gig.
> >
> > i've had these problems on machines as "small" as 8GiB. the real problem
> > is that the kernel will let millions of potential (write) IO ops stack up
> > for a device which can handle only mere 100s of IOs per second. (and i'm
> > not convinced it does the IOs in a sane order when it has millions to
> > choose from)
> >
> > replacing the percentage based dirty_ratio / dirty_background_ratio with
> > sane kibibyte units is a good fix... but i'm not sure it's sufficient.
> >
> > it seems like the "flow control" mechanism (i.e. dirty_ratio) should be on
> > a device basis...
> >
> > try running doug ledford'd memtest.sh on an 8GiB box with a single disk,
> > let it go a few minutes then ^C and type "sync". i've had to wait 10
> > minutes (2.6.18 with default vm settings).
> >
> > it makes it hard to guarantee a box can shutdown quickly -- nasty for
> > setting up UPS on-battery timeouts for example.
> >
>
> Increasing the request queue size should help there
> (/sys/block/sda/queue/nr_requests). Maybe 25% or more benefit with that
> test, at a guess.
hmm i've never had much luck with increasing nr_requests... if i get a
chance i'll reproduce the problem and try that.
> Probably initscripts should do that rather than leaving the kernel defaults
> in place. It's a bit tricky for the kernel to do because the decision
> depends upon the number of disks in the system, as well as the amount of
> memory.
>
> Or perhaps the kernel should implement a system-wide limit on the number of
> requests in flight. While avoiding per-device starvation. Tricky.
actually a global dirty_ratio causes interference between devices which
should otherwise not block each other...
if you set up a "dd if=/dev/zero of=/dev/sdb bs=1M" it shouldn't affect
write performance on sda -- but it does... because the dd basically
dirties all of the "dirty_background_ratio" pages and then any task
writing to sda has to block in the foreground... (i've had this happen in
practice -- my hack fix is oflag=direct on the dd... but the problem still
exists.)
i'm not saying fixing any of this is easy, i'm just being a user griping
about it :)
-dean
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