Messages in this thread | | | Date | Wed, 24 Nov 2010 22:38:18 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 06/13] writeback: bdi write bandwidth estimation |
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On Wed, Nov 24, 2010 at 10:31:57PM +0800, Peter Zijlstra wrote: > 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.
Good point!
> Also, it increases the memory footprint of the whole solution.
Yeah, maybe not a good trade off.
> > > +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.
When there are lots of concurrent writers, we'll target at ~100ms pause time, hence the update frequency will be lowered accordingly.
Thanks, Fengguang
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