Messages in this thread | | | Subject | Re: [PATCH 06/13] writeback: bdi write bandwidth estimation | From | Peter Zijlstra <> | Date | Wed, 24 Nov 2010 15:31:57 +0100 |
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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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