Re: [PATCH v3 RFC 14/14] mm: speedup page alloc for MPOL_PREFERRED_MANY by adding a NO_SLOWPATH gfp bit

[Feng Tang]

Hi Michal,

On Wed, Mar 03, 2021 at 12:39:57PM +0100, Michal Hocko wrote:
> On Wed 03-03-21 18:20:58, Feng Tang wrote:
> > When doing broader test, we noticed allocation slowness in one test
> > case that malloc memory with size which is slightly bigger than free
> > memory of targeted nodes, but much less then the total free memory
> > of system.
> > 
> > The reason is the code enters the slowpath of __alloc_pages_nodemask(),
> > which takes quite some time. As alloc_pages_policy() will give it a 2nd
> > try with NULL nodemask, so there is no need to enter the slowpath for
> > the first try. Add a new gfp bit to skip the slowpath, so that user cases
> > like this can leverage.
> > 
> > With it, the malloc in such case is much accelerated as it never enters
> > the slowpath.
> > 
> > Adding a new gfp_mask bit is generally not liked, and another idea is to
> > add another nodemask to struct 'alloc_context', so it has 2: 'preferred-nmask'
> > and 'fallback-nmask', and they will be tried in turn if not NULL, with
> > it we can call __alloc_pages_nodemask() only once.
> 
> Yes, it is very much disliked. Is there any reason why you cannot use
> GFP_NOWAIT for that purpose?

I did try that at the first place, but it didn't obviously change the slowness.
I assumed the direct claim was still involved as GFP_NOWAIT only impact kswapd
reclaim.

Thanks,
Feng


> -- 
> Michal Hocko
> SUSE Labs