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

[Michal Hocko]

On Fri 15-05-20 16:02:10, Feng Tang wrote:
> 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.

Yeah, I have misread that part. Sorry about that.

[...]
> > > +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)

Try to measure those and see what numbers look like.
-- 
Michal Hocko
SUSE Labs