Re: [Lsf-pc] [LSF/MM/BPF TOPIC] reducing direct map fragmentation

[Michal Hocko <mhocko@suse.com>]

On Fri 21-04-23 12:47:06, Mike Rapoport wrote:
> On Fri, Apr 21, 2023 at 11:05:20AM +0200, Michal Hocko wrote:
> > Hi,
> > 
> > On Wed 01-02-23 20:06:37, Mike Rapoport wrote:
> > [...]
> > > My current proposal is to have a cache of 2M pages close to the page
> > > allocator and use a GFP flag to make allocation request use that cache. On
> > > the free() path, the pages that are mapped at PTE level will be put into
> > > that cache.
> > 
> > Are there stil open questions which would benefit from a discussion at
> > LSFMM this year?
> 
> Yes, I believe.
> 
> I was trying to get some numbers to see what would be the benefit of
> __GFP_UNMAPPED and I couldn't find a benchmark that will produce results
> with good signal-to-noise ratio.
> 
> So while it seems that there's a general agreement on how to implement
> caching of 2M pages, there is still no evidence that it will be universally
> useful. 
> 
> It would be interesting to discuss the reasons for inconclusive results,
> and more importantly, what should be the general direction for dealing with
> the direct map fragmentation.
> 
> As it seems now, packing code allocations into 2M pages would be an
> improvement, while data allocations that fragment the direct map do not
> impact much the overall system performance.
> 
> I'll bring the mmtest results I have to begin the discussion.

Makes sense. Thanks!
-- 
Michal Hocko
SUSE Labs