[ovs-dev] [PATCH 06/12] cmap: Remove prefetching in cmap_find_batch().

[Jarno Rajahalme]

> On Oct 7, 2016, at 9:17 AM, Bhanuprakash Bodireddy <bhanuprakash.bodireddy at intel.com> wrote:
> 
> prefetching the data in to the caches isn't improving the performance in
> cmap_find_batch(). Moreover its found that there is slight improvement
> in performance with out prefetching.
> 

I recall doing some performance testing for this earlier, but have no records of the system or other circumstances. Is it likely that this is at least somewhat system and test case dependent? Also the modified batch size may be a factor here. Anyway, I don’t currently have any proof to the contrary, so I have no problem removing the prefetching.

However, you should also update all the comments referring to prefetching.

  Jarno

> This patch removes prefetching from cmap_find_batch().
> 
> Signed-off-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy at intel.com>
> Signed-off-by: Antonio Fischetti <antonio.fischetti at intel.com>
> ---
> lib/cmap.c | 4 ----
> 1 file changed, 4 deletions(-)
> 
> diff --git a/lib/cmap.c b/lib/cmap.c
> index 8c7312d..4c34bda 100644
> --- a/lib/cmap.c
> +++ b/lib/cmap.c
> @@ -397,7 +397,6 @@ cmap_find_batch(const struct cmap *cmap, unsigned long map,
>     ULLONG_FOR_EACH_1(i, map) {
>         h1s[i] = rehash(impl, hashes[i]);
>         b1s[i] = &impl->buckets[h1s[i] & impl->mask];
> -        OVS_PREFETCH(b1s[i]);
>     }
>     /* Lookups, Round 1. Only look up at the first bucket. */
>     ULLONG_FOR_EACH_1(i, map) {
> @@ -413,13 +412,11 @@ cmap_find_batch(const struct cmap *cmap, unsigned long map,
>         if (!node) {
>             /* Not found (yet); Prefetch the 2nd bucket. */
>             b2s[i] = &impl->buckets[other_hash(h1s[i]) & impl->mask];
> -            OVS_PREFETCH(b2s[i]);
>             c1s[i] = c1; /* We may need to check this after Round 2. */
>             continue;
>         }
>         /* Found. */
>         ULLONG_SET0(map, i); /* Ignore this on round 2. */
> -        OVS_PREFETCH(node);
>         nodes[i] = node;
>     }
>     /* Round 2. Look into the 2nd bucket, if needed. */
> @@ -453,7 +450,6 @@ cmap_find_batch(const struct cmap *cmap, unsigned long map,
>             continue;
>         }
> found:
> -        OVS_PREFETCH(node);
>         nodes[i] = node;
>     }
>     return result;
> -- 
> 2.4.11
> 
> _______________________________________________
> dev mailing list
> dev at openvswitch.org
> http://openvswitch.org/mailman/listinfo/dev