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    SubjectRe: [PATCH] O11int for interactivity
    On Tue, 05 Aug 2003 13:38:34 +1000, Nick Piggin said:

    > Of course some minimum read _latency_ would be required: this
    > could actually be done easily with strace come to think of it.
    > Maybe some xmms mapped memory is being swapped out? But that
    > would be more of a VM problem.

    I was seeing some CPU-related pauses, but once Con's work got to O7 or so,
    those disappeared. I'm *quite* convinced that the remaining glitches
    are VM related, mostly because every glitch seems to be associated with
    an increase in the 'pswpout' field in /proc/vmstat (yes, I tested with stuff like
    "for (;;) do cat /proc/vmstat; sleep 1 done;".

    The *odd* part is that the pgpgin, pgpgout, and pswpin numbers do *NOT*
    seem to be correlated. High I/O loads from read/write don't seem to cause
    a problem - untarring the Linux distro won't do it, running badblocks won't do it.

    But if somebody has to swap out, all hell breaks loose...

    Hmm.. looking at mm/page_io.c, it seems swap_writepage() calls get_swap_bio
    with GFP_NOIO, while readdpage() uses GFP_KERNEL. I wonder if that GFP_NOIO is
    causing ugliness - that's really __GFP_WAIT, and the comments in bio_alloc() are
    pretty clear that it can block. And remember we're not getting into this code unless
    we're already under memory pressure....

    (And if somebody tells me how to instrument a -test2-mm4 kernel so I can tell if
    I'm on crack or not, I'll happily do so....)
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