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    SubjectRe: Likley stupid question on "throttle_vm_writeout"
    ----- Original Message ----

    > From: Wu Fengguang <fengguang.wu@intel.com>
    > To: Peter Zijlstra <peterz@infradead.org>
    > Cc: Martin Knoblauch <spamtrap@knobisoft.de>; linux-kernel@vger.kernel.org
    > Sent: Tue, November 10, 2009 3:08:58 AM
    > Subject: Re: Likley stupid question on "throttle_vm_writeout"
    >
    > On Mon, Nov 09, 2009 at 04:26:33PM +0100, Peter Zijlstra wrote:
    > > On Mon, 2009-11-09 at 07:15 -0800, Martin Knoblauch wrote:
    > > > Hi, (please CC me on replies)
    > > >
    > > > I have a likely stupid question on the function "throttle_vm_writeout".
    > Looking at the code I find:
    > > >
    > > > if (global_page_state(NR_UNSTABLE_NFS) +
    > > > global_page_state(NR_WRITEBACK) <= dirty_thresh)
    > > > break;
    > > > congestion_wait(WRITE, HZ/10);
    > > >
    > > > Shouldn't the NR_FILE_DIRTY pages be considered as well?
    > >
    > > Ha, you just trod onto a piece of ugly I'd totally forgotten about ;-)
    > >
    > > The intent of throttle_vm_writeout() is to limit the total pages in
    > > writeout and to wait for them to go-away.
    >
    > Like this:
    >
    > vmscan fast => large NR_WRITEBACK => throttle vmscan based on it
    >
    > > Everybody hates the function, nobody managed to actually come up with
    > > anything better.
    >
    > btw, here is another reason to limit NR_WRITEBACK: I saw many
    > throttle_vm_writeout() waits if there is no wait queue to limit
    > NR_WRITEBACK (eg. NFS). In that case the (steadily) big NR_WRITEBACK
    > is _not_ caused by fast vmscan..
    >

    That is exactely what made me look again into the code. My observation is that when doing something like:

    dd if=/dev/zero of=fast-local-disk bs=1M count=15000

    most of the "dirty" pages are in NR_FILE_DIRTY with some relatively small amount (10% or so) in NR_WRITEBACK. If I do:

    dd if=/dev/zero of=some-nfs-mount bs=1M count=15000

    NR_WRITEBACK almost immediatelly goes up to dirty_ratio, with NR_UNSTABLE_NFS small. Over time NR_UNSTABLE_NFS grows, but is always lower than NR_WRITEBACK (maybe 40/60).

    But don't ask what happens if I do both in parallel.... The local IO really slows to a crawl and sometimes the system just becomes very unresponsive. Have we heard that before? :-)

    Somehow I have the impression that NFS writeout is able to absolutely dominate the dirty pages to an extent that the system is unusable.

    Cheers
    Martin



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