Re: [PATCH v3 3/3] vfs: Use per-cpu list for superblock's inode list
From: Ingo Molnar
Date: Thu Feb 25 2016 - 03:06:45 EST
* Jan Kara <jack@xxxxxxx> 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?