On 02/25/2016 03:06 AM, Ingo Molnar wrote: > * Jan Kara wrote: > >>>>> With an exit microbenchmark that creates a large number of threads, >>>>> attachs many inodes to them and then exits. The runtimes of that >>>>> microbenchmark with 1000 threads before and after the patch on a 4-socket >>>>> Intel E7-4820 v3 system (40 cores, 80 threads) were as follows: >>>>> >>>>> Kernel Elapsed Time System Time >>>>> ------ ------------ ----------- >>>>> Vanilla 4.5-rc4 65.29s 82m14s >>>>> Patched 4.5-rc4 22.81s 23m03s >>>>> >>>>> Before the patch, spinlock contention at the inode_sb_list_add() function >>>>> at the startup phase and the inode_sb_list_del() function at the exit >>>>> phase were about 79% and 93% of total CPU time respectively (as measured >>>>> by perf). After the patch, the percpu_list_add() function consumed only >>>>> about 0.04% of CPU time at startup phase. The percpu_list_del() function >>>>> consumed about 0.4% of CPU time at exit phase. There were still some >>>>> spinlock contention, but they happened elsewhere. >>>> While looking through this patch, I have noticed that the >>>> list_for_each_entry_safe() iterations in evict_inodes() and >>>> invalidate_inodes() are actually unnecessary. So if you first apply the >>>> attached patch, you don't have to implement safe iteration variants at all. >>>> >>>> As a second comment, I'd note that this patch grows struct inode by 1 >>>> pointer. It is probably acceptable for large machines given the speedup but >>>> it should be noted in the changelog. Furthermore for UP or even small SMP >>>> systems this is IMHO undesired bloat since the speedup won't be noticeable. >>>> >>>> So for these small systems it would be good if per-cpu list magic would just >>>> fall back to single linked list with a spinlock. Do you think that is >>>> reasonably doable? >>> Even many 'small' systems tend to be SMP these days. >> Yes, I know. But my tablet with 4 ARM cores is unlikely to benefit from this >> change either. [...] > I'm not sure about that at all, the above numbers are showing a 3x-4x speedup in > system time, which ought to be noticeable on smaller SMP systems as well. > > Waiman, could you please post the microbenchmark? > > Thanks, > > Ingo The microbenchmark that I used is attached. I do agree that performance benefit will decrease as the number of CPUs get smaller. The system that I used for testing have 4 sockets with 40 cores (80 threads). Dave Chinner had run his fstests on a 16-core system (probably 2-socket) which showed modest improvement in performance (~4m40s vs 4m30s in runtime). This patch enables parallel insertion and deletion to/from the inode list which used to be a serialized operation. So if that list operation is a bottleneck, you will see significant improvement. If it is not, we may not notice that much of a difference. For a single-socket 4-core system, I agree that the performance benefit, if any, will be limited. Cheers, Longman