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    SubjectRe: [RFC][PATCH 0/6] per device dirty throttling
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    On Tue, 2007-03-20 at 20:38 +1100, David Chinner wrote:
    > On Tue, Mar 20, 2007 at 09:08:24AM +0100, Peter Zijlstra wrote:
    > > On Tue, 2007-03-20 at 18:47 +1100, David Chinner wrote:
    > > > So overall we've lost about 15-20% of the theoretical aggregate
    > > > perfomrance, but we haven't starved any of the devices over a
    > > > long period of time.
    > > >
    > > > However, looking at vmstat for total throughput, there are periods
    > > > of time where it appears that the fastest disk goes idle. That is,
    > > > we drop from an aggregate of about 550MB/s to below 300MB/s for
    > > > several seconds at a time. You can sort of see this from the file
    > > > size output above - long term the ratios remain the same, but in the
    > > > short term we see quite a bit of variability.
    > >
    > > I suspect you did not apply 7/6? There is some trouble with signed vs
    > > unsigned in the initial patch set that I tried to 'fix' by masking out
    > > the MSB, but that doesn't work and results in 'time' getting stuck for
    > > about half the time.
    >
    > I applied the fixes patch as well, so i had all that you posted...

    Humm, not that then.

    > > > but it's almost
    > > > like it is throttling a device completely while it allows another
    > > > to finish writing it's quota (underestimating bandwidth?).
    > >
    > > Yeah, there is some lumpy-ness in BIO submission or write completions it
    > > seems, and when that granularity (multiplied by the number of active
    > > devices) is larger than the 'time' period over with we average
    > > (indicated by vm_cycle_shift) very weird stuff can happen.
    >
    > Sounds like the period is a bit too short atm if we can get into this
    > sort of problem with only 2 active devices....

    Yeah, trouble is, I significantly extended this period in 7/6.
    Will have to ponder a bit on what is happening then.

    Anyway, thanks for the feedback.

    I'll try and reproduce the umount problem, maybe that will give some
    hints.

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