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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html