Re: alloc_pages_bulk()

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Feb 11, 2021 at 04:20:31PM +0000, Chuck Lever wrote:
> > On Feb 11, 2021, at 4:12 AM, Mel Gorman <mgorman@techsingularity.net> wrote:
> > 
> > <SNIP>
> > 
> > Parameters to __rmqueue_pcplist are garbage as the parameter order changed.
> > I'm surprised it didn't blow up in a spectacular fashion. Again, this
> > hasn't been near any testing and passing a list with high orders to
> > free_pages_bulk() will corrupt lists too. Mostly it's a curiousity to see
> > if there is justification for reworking the allocator to fundamentally
> > deal in batches and then feed batches to pcp lists and the bulk allocator
> > while leaving the normal GFP API as single page "batches". While that
> > would be ideal, it's relatively high risk for regressions. There is still
> > some scope for adding a basic bulk allocator before considering a major
> > refactoring effort.
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index f8353ea7b977..8f3fe7de2cf7 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5892,7 +5892,7 @@ __alloc_pages_bulk_nodemask(gfp_t gfp_mask, unsigned int order,
> > 	pcp_list = &pcp->lists[migratetype];
> > 
> > 	while (nr_pages) {
> > -		page = __rmqueue_pcplist(zone, gfp_mask, migratetype,
> > +		page = __rmqueue_pcplist(zone, migratetype, alloc_flags,
> > 								pcp, pcp_list);
> > 		if (!page)
> > 			break;
> 
> The NFS server is considerably more stable now. Thank you!
> 

Thanks for testing!

> I confirmed that my patch is requesting and getting multiple pages.
> The new NFSD code and the API seem to be working as expected.
> 
> The results are stunning. Each svc_alloc_arg() call here allocates
> 65 pages to satisfy a 256KB NFS READ request.
> 
> Before:
> 
>             nfsd-972   [000]   584.513817: funcgraph_entry:      + 35.385 us  |  svc_alloc_arg();
>             nfsd-979   [002]   584.513870: funcgraph_entry:      + 29.051 us  |  svc_alloc_arg();
>             nfsd-980   [001]   584.513951: funcgraph_entry:      + 29.178 us  |  svc_alloc_arg();
>             nfsd-983   [000]   584.514014: funcgraph_entry:      + 29.211 us  |  svc_alloc_arg();
>             nfsd-976   [002]   584.514059: funcgraph_entry:      + 29.315 us  |  svc_alloc_arg();
>             nfsd-974   [001]   584.514127: funcgraph_entry:      + 29.237 us  |  svc_alloc_arg();
> 
> After:
> 
>             nfsd-977   [002]    87.049425: funcgraph_entry:        4.293 us   |  svc_alloc_arg();
>             nfsd-981   [000]    87.049478: funcgraph_entry:        4.059 us   |  svc_alloc_arg();
>             nfsd-988   [001]    87.049549: funcgraph_entry:        4.474 us   |  svc_alloc_arg();
>             nfsd-983   [003]    87.049612: funcgraph_entry:        3.819 us   |  svc_alloc_arg();
>             nfsd-976   [000]    87.049619: funcgraph_entry:        3.869 us   |  svc_alloc_arg();
>             nfsd-980   [002]    87.049738: funcgraph_entry:        4.124 us   |  svc_alloc_arg();
>             nfsd-975   [000]    87.049769: funcgraph_entry:        3.734 us   |  svc_alloc_arg();
> 

Uhhhh, that is much better than I expected given how lame the
implementation is. Sure -- it works, but it has more overhead than it
should with the downside that reducing it requires fairly deep surgery. It
may be enough to tidy this up to handle order-0 pages only to start with
and see how far it gets. That's a fairly trivial modification.

> There appears to be little cost change for single-page allocations
> using the bulk allocator (nr_pages=1):
> 
> Before:
> 
>             nfsd-985   [003]   572.324517: funcgraph_entry:        0.332 us   |  svc_alloc_arg();
>             nfsd-986   [001]   572.324531: funcgraph_entry:        0.311 us   |  svc_alloc_arg();
>             nfsd-985   [003]   572.324701: funcgraph_entry:        0.311 us   |  svc_alloc_arg();
>             nfsd-986   [001]   572.324727: funcgraph_entry:        0.424 us   |  svc_alloc_arg();
>             nfsd-985   [003]   572.324760: funcgraph_entry:        0.332 us   |  svc_alloc_arg();
>             nfsd-986   [001]   572.324786: funcgraph_entry:        0.390 us   |  svc_alloc_arg();
> 
> After:
> 
>             nfsd-989   [002]    75.043226: funcgraph_entry:        0.322 us   |  svc_alloc_arg();
>             nfsd-988   [001]    75.043436: funcgraph_entry:        0.368 us   |  svc_alloc_arg();
>             nfsd-989   [002]    75.043464: funcgraph_entry:        0.424 us   |  svc_alloc_arg();
>             nfsd-988   [001]    75.043490: funcgraph_entry:        0.317 us   |  svc_alloc_arg();
>             nfsd-989   [002]    75.043517: funcgraph_entry:        0.425 us   |  svc_alloc_arg();
>             nfsd-988   [001]    75.050025: funcgraph_entry:        0.407 us   |  svc_alloc_arg();
> 

That is not too surprising given that there would be some additional
overhead to manage a list of 1 page. I would hope that users of the bulk
allocator are not routinely calling it with nr_pages == 1.

-- 
Mel Gorman
SUSE Labs