On Tue, Jun 22, 2010 at 10:02:59PM +0800, Jan Kara wrote: > On Tue 22-06-10 21:52:34, Wu Fengguang wrote: > > > On the other hand I think we will have to come up with something > > > more clever than what I do now because for some huge machines with > > > nr_cpu_ids == 256, the error of the counter is 256*9*8 = 18432 so that's > > > already unacceptable given the amounts we want to check (like 1536) - > > > already for nr_cpu_ids == 32, the error is the same as the difference we > > > want to check. I think we'll have to come up with some scheme whose error > > > is not dependent on the number of cpus or if it is dependent, it's only a > > > weak dependency (like a logarithm or so). > > > Or we could rely on the fact that IO completions for a bdi won't happen on > > > all CPUs and thus the error would be much more bounded. But I'm not sure > > > how much that is true or not. > > > > Yes the per CPU counter seems tricky. How about plain atomic operations? > > > > This test shows that atomic_dec_and_test() is about 4.5 times slower > > than plain i-- in a 4-core CPU. Not bad. > > > > Note that > > 1) we can avoid the atomic operations when there are no active waiters > > 2) most writeback will be submitted by one per-bdi-flusher, so no worry > > of cache bouncing (this also means the per CPU counter error is > > normally bounded by the batch size) > Yes, writeback will be submitted by one flusher thread but the question > is rather where the writeback will be completed. And that depends on which > CPU that particular irq is handled. As far as my weak knowledge of HW goes, > this very much depends on the system configuration (i.e., irq affinity and > other things). Either the irq goes to the io submit CPU, or some fixed CPU (somehow determined by the bdi?) I guess? My wild guess is, it may be bad for the irq to goto some random CPU... Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>