On Mon, 28 Sep 2015 07:53:16 -0700 Alexander Duyck <alexander.du...@gmail.com> wrote:
> On 09/28/2015 05:26 AM, Jesper Dangaard Brouer wrote: > > For practical use-cases it is beneficial to prefetch the next freelist > > object in bulk allocation loop. > > > > Micro benchmarking show approx 1 cycle change: > > > > bulk - prev-patch - this patch > > 1 - 49 cycles(tsc) - 49 cycles(tsc) - increase in cycles:0 > > 2 - 30 cycles(tsc) - 31 cycles(tsc) - increase in cycles:1 > > 3 - 23 cycles(tsc) - 25 cycles(tsc) - increase in cycles:2 > > 4 - 20 cycles(tsc) - 22 cycles(tsc) - increase in cycles:2 > > 8 - 18 cycles(tsc) - 19 cycles(tsc) - increase in cycles:1 > > 16 - 17 cycles(tsc) - 18 cycles(tsc) - increase in cycles:1 > > 30 - 18 cycles(tsc) - 17 cycles(tsc) - increase in cycles:-1 > > 32 - 18 cycles(tsc) - 19 cycles(tsc) - increase in cycles:1 > > 34 - 23 cycles(tsc) - 24 cycles(tsc) - increase in cycles:1 > > 48 - 21 cycles(tsc) - 22 cycles(tsc) - increase in cycles:1 > > 64 - 20 cycles(tsc) - 21 cycles(tsc) - increase in cycles:1 > > 128 - 27 cycles(tsc) - 27 cycles(tsc) - increase in cycles:0 > > 158 - 30 cycles(tsc) - 30 cycles(tsc) - increase in cycles:0 > > 250 - 37 cycles(tsc) - 37 cycles(tsc) - increase in cycles:0 > > > > Note, benchmark done with slab_nomerge to keep it stable enough > > for accurate comparison. > > > > Signed-off-by: Jesper Dangaard Brouer <bro...@redhat.com> > > --- > > mm/slub.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/mm/slub.c b/mm/slub.c > > index c25717ab3b5a..5af75a618b91 100644 > > --- a/mm/slub.c > > +++ b/mm/slub.c > > @@ -2951,6 +2951,7 @@ bool kmem_cache_alloc_bulk(struct kmem_cache *s, > > gfp_t flags, size_t size, > > goto error; > > > > c = this_cpu_ptr(s->cpu_slab); > > + prefetch_freepointer(s, c->freelist); > > continue; /* goto for-loop */ > > } > > > > @@ -2960,6 +2961,7 @@ bool kmem_cache_alloc_bulk(struct kmem_cache *s, > > gfp_t flags, size_t size, > > goto error; > > > > c->freelist = get_freepointer(s, object); > > + prefetch_freepointer(s, c->freelist); > > p[i] = object; > > > > /* kmem_cache debug support */ > > > > I can see the prefetch in the last item case being possibly useful since > you have time between when you call the prefetch and when you are > accessing the next object. However, is there any actual benefit to > prefetching inside the loop itself? Based on your data above it doesn't > seem like that is the case since you are now adding one additional cycle > to the allocation and I am not seeing any actual gain reported here. The gain will first show up, when using bulk alloc in real use-cases. As you know, bulk alloc on RX path don't show any improvement. And I measured (with perf-mem-record) L1 miss'es here. I could reduce the L1 misses here by adding prefetch. But I cannot remember if I measured any PPS improvement with this. As you hint, the time I have between my prefetch and use is very small, thus the question is if this will show any benefit for real use-cases. We can drop this patch, and then I'll include it in my network use-case, and measure the effect? (Although I'll likely be wasting my time, as we should likely redesign the alloc API instead). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
