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    SubjectRe: [PATCH 06/13] writeback: bdi write bandwidth estimation
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    On Wed, 2010-11-24 at 22:21 +0800, Wu Fengguang wrote:
    >
    > Hmm, but why not avoid locking at all? With per-cpu bandwidth vars,
    > each CPU will see slightly different bandwidth, but that should be
    > close enough and not a big problem.

    I don't think so, on a large enough machine some cpus might hardly ever
    use a particular BDI and hence get very stale data.

    Also, it increases the memory footprint of the whole solution.

    > > +void bdi_update_write_bandwidth(struct backing_dev_info *bdi)
    > > +{
    > > + unsigned long time_now, write_now;
    > > + long time_delta, write_delta;
    > > + long bw;
    > > +
    > > + if (!spin_try_lock(&bdi->bw_lock))
    > > + return;
    >
    > spin_try_lock is good, however is still global state and risks
    > cacheline bouncing..

    If there are many concurrent writers to the BDI I don't think this is
    going to be the top sore spot, once it is we can think of something
    else.


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