> On Feb 15, 2021, at 7:06 AM, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Feb 11, 2021 at 04:20:31PM +0000, Chuck Lever wrote:
>>> On Feb 11, 2021, at 4:12 AM, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> 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.
My experience with function tracing is the entry and exit
timestamping adds significant overhead. I'd bet the actual
timing improvement is still good but much less.
> 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.
I'd like to see an "order-0 only" implementation go in soon.
The improvement is palpable and there are several worthy
consumers on deck.
>> 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.
The NFSD implementation I did uses alloc_pages_bulk() to fill however
many pages are needed. Often that's just one page.
Sometimes it's zero pages. alloc_pages_bulk() does not behave very
well, so NFSD avoids calling it in that case.
I can post the patch for review. I cleaned it up recently but haven't
had a chance to test the clean-ups, so it might not work in its
current state.
--
Chuck Lever
