Re: [PATCH 3/3] mm: adjust vm_committed_as_batch according to vm overcommit policy

[Feng Tang]

Hi Michal,

Thanks for the thorough reviews for these 3 patches!

On Fri, May 15, 2020 at 03:41:25PM +0800, Michal Hocko wrote:
> On Fri 08-05-20 15:25:17, Feng Tang wrote:
> > When checking a performance change for will-it-scale scalability
> > mmap test [1], we found very high lock contention for spinlock of
> > percpu counter 'vm_committed_as':
> > 
> >     94.14%     0.35%  [kernel.kallsyms]         [k] _raw_spin_lock_irqsave
> >     48.21% _raw_spin_lock_irqsave;percpu_counter_add_batch;__vm_enough_memory;mmap_region;do_mmap;
> >     45.91% _raw_spin_lock_irqsave;percpu_counter_add_batch;__do_munmap;
> > 
> > Actually this heavy lock contention is not always necessary. The
> > 'vm_committed_as' needs to be very precise when the strict
> > OVERCOMMIT_NEVER policy is set, which requires a rather small batch
> > number for the percpu counter.
> > 
> > So lift the batch number to 16X for OVERCOMMIT_ALWAYS and
> > OVERCOMMIT_GUESS policies, and add a sysctl handler to adjust it
> > when the policy is reconfigured.
> 
> Increasing the batch size for weaker overcommit modes makes sense. But
> your patch is also tuning OVERCOMMIT_NEVER without any explanation why
> that is still "small enough to be precise".

Actually, it keeps the batch algorithm for "OVERCOMMIT_NEVER", but
change the other 2 policies, which I should set it clear in the
commit log.

> > Benchmark with the same testcase in [1] shows 53% improvement on a
> > 8C/16T desktop, and 2097%(20X) on a 4S/72C/144T server. And no change
> > for some platforms, due to the test mmap size of the case is bigger
> > than the batch number computed, though the patch will help mmap/munmap
> > generally.
> > 
> > [1] https://lkml.org/lkml/2020/3/5/57
> 
> Please do not use lkml.org links in the changelog. Use
> http://lkml.kernel.org/r/$msg instead.

Thanks, will keep that in mind for this and future patches.

> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> >  s32 vm_committed_as_batch = 32;
> >  
> > -static void __meminit mm_compute_batch(void)
> > +void mm_compute_batch(void)
> >  {
> >  	u64 memsized_batch;
> >  	s32 nr = num_present_cpus();
> >  	s32 batch = max_t(s32, nr*2, 32);
> > -
> > -	/* batch size set to 0.4% of (total memory/#cpus), or max int32 */
> > -	memsized_batch = min_t(u64, (totalram_pages()/nr)/256, 0x7fffffff);
> > +	unsigned long ram_pages = totalram_pages();
> > +
> > +	/*
> > +	 * For policy of OVERCOMMIT_NEVER, set batch size to 0.4%
> > +	 * of (total memory/#cpus), and lift it to 6.25% for other
> > +	 * policies to easy the possible lock contention for percpu_counter
> > +	 * vm_committed_as, while the max limit is INT_MAX
> > +	 */
> > +	if (sysctl_overcommit_memory == OVERCOMMIT_NEVER)
> > +		memsized_batch = min_t(u64, ram_pages/nr/256, INT_MAX);
> > +	else
> > +		memsized_batch = min_t(u64, ram_pages/nr/16, INT_MAX);

Also as you mentioned there are real-world work loads with big mmap
size and multi-threading, can we lift it even further ?
	memsized_batch = min_t(u64, ram_pages/nr/4, INT_MAX)

Thanks,
Feng