On Wed, Apr 01, 2009 at 07:28:16AM +1100, Greg Banks wrote:
> Use the IPv4 address of the client in the reply cache hash function.
> This can help improve the distribution of the hash function when the
> workload includes a large number of clients which mounted their NFS
> filesystems at nearly the same time and are doing similar sequences
> of NFS calls, a pattern seen with large compute clusters.
>
> This code predates the IPv6 support in the current NFS server but
> should be harmless with IPv6 clients.
>
> Signed-off-by: Greg Banks <gnb@xxxxxxx>
> ---
>
> fs/nfsd/nfscache.c | 27 +++++++++++++--------------
> 1 file changed, 13 insertions(+), 14 deletions(-)
>
> Index: bfields/fs/nfsd/nfscache.c
> ===================================================================
> --- bfields.orig/fs/nfsd/nfscache.c
> +++ bfields/fs/nfsd/nfscache.c
> @@ -38,12 +38,17 @@ static int cache_disabled = 1;
> * Calculate the hash index from an XID. Note, some clients increment
> * their XIDs in host order, which can result in all the variation being
> * in the top bits we see here. So we fold those bits down.
> + *
> + * Experiment shows that using the Jenkins hash improves the spectral
> + * properties of this hash, but the CPU cost of calculating it outweighs
> + * the advantages.
> */
> -static inline u32 request_hash(u32 xid)
> +static inline u32 request_hash(u32 xid, const struct sockaddr_in *sin)
> {
> u32 h = xid;
> h ^= (xid >> 24);
> h ^= ((xid & 0xff0000) >> 8);
> + h ^= sin->sin_addr.s_addr;
Tell me if I'm confused about the endianness: the variation is typically
in the low-order (host) end of the ip address, but the s_addr is stored
in network order, so the variation is in the high-order bits on a
little-endian machine, but &(HASHSIZE-1) is throwing out those bits.
> return h & (HASHSIZE-1);
> }
>
I'd've stuck the following in a separate patch as it's not really
related.
--b.
> @@ -114,16 +119,6 @@ lru_put_end(struct svc_cacherep *rp)
> }
>
> /*
> - * Move a cache entry from one hash list to another
> - */
> -static void
> -hash_refile(struct svc_cacherep *rp)
> -{
> - hlist_del_init(&rp->c_hash);
> - hlist_add_head(&rp->c_hash, cache_hash + request_hash(rp->c_xid));
> -}
> -
> -/*
> * Try to find an entry matching the current call in the cache. When none
> * is found, we grab the oldest unlocked entry off the LRU list.
> * Note that no operation within the loop may sleep.
> @@ -137,7 +132,8 @@ nfsd_cache_lookup(struct svc_rqst *rqstp
> __be32 xid = rqstp->rq_xid;
> u32 proto = rqstp->rq_prot,
> vers = rqstp->rq_vers,
> - proc = rqstp->rq_proc;
> + proc = rqstp->rq_proc,
> + h;
> unsigned long age;
> int rtn;
>
> @@ -146,11 +142,12 @@ nfsd_cache_lookup(struct svc_rqst *rqstp
> nfsdstats.rcnocache++;
> return RC_DOIT;
> }
> + h = request_hash(xid, svc_addr_in(rqstp));
>
> spin_lock(&cache_lock);
> rtn = RC_DOIT;
>
> - rh = &cache_hash[request_hash(xid)];
> + rh = &cache_hash[h];
> hlist_for_each_entry(rp, hn, rh, c_hash) {
> if (rp->c_state != RC_UNUSED &&
> xid == rp->c_xid && proc == rp->c_proc &&
> @@ -198,7 +195,9 @@ nfsd_cache_lookup(struct svc_rqst *rqstp
> rp->c_vers = vers;
> rp->c_timestamp = jiffies;
>
> - hash_refile(rp);
> + /* Move the cache entry from one hash list to another */
> + hlist_del_init(&rp->c_hash);
> + hlist_add_head(&rp->c_hash, cache_hash + h);
>
> /* release any buffer */
> if (rp->c_type == RC_REPLBUFF) {
>
> --
> Greg
--
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