From 98d821bda189ba2bfcb3877ea3064da3403698ae Mon Sep 17 00:00:00 2001 From: Jeff Layton Date: Wed, 27 Mar 2013 10:15:39 -0400 Subject: [PATCH] nfsd: keep stats on worst hash balancing seen so far The typical case with the DRC is a cache miss, so if we keep track of the max number of entries that we've ever walked over in a search, then we should have a reasonable estimate of the longest hash chain that we've ever seen. With that, we'll also keep track of the total size of the cache when we see the longest chain. In the case of a tie, we prefer to track the smallest total cache size in order to properly gauge the worst-case ratio of max vs. avg chain length. Signed-off-by: Jeff Layton Signed-off-by: J. Bruce Fields --- fs/nfsd/nfscache.c | 31 +++++++++++++++++++++++++++---- 1 file changed, 27 insertions(+), 4 deletions(-) diff --git a/fs/nfsd/nfscache.c b/fs/nfsd/nfscache.c index fd81ca79a002..17cb0d6b9944 100644 --- a/fs/nfsd/nfscache.c +++ b/fs/nfsd/nfscache.c @@ -41,6 +41,12 @@ static unsigned int payload_misses; /* amount of memory (in bytes) currently consumed by the DRC */ static unsigned int drc_mem_usage; +/* longest hash chain seen */ +static unsigned int longest_chain; + +/* size of cache when we saw the longest hash chain */ +static unsigned int longest_chain_cachesize; + /* * Calculate the hash index from an XID. */ @@ -319,15 +325,30 @@ nfsd_cache_match(struct svc_rqst *rqstp, __wsum csum, struct svc_cacherep *rp) static struct svc_cacherep * nfsd_cache_search(struct svc_rqst *rqstp, __wsum csum) { - struct svc_cacherep *rp; + struct svc_cacherep *rp, *ret = NULL; struct hlist_head *rh; + unsigned int entries = 0; rh = &cache_hash[request_hash(rqstp->rq_xid)]; hlist_for_each_entry(rp, rh, c_hash) { - if (nfsd_cache_match(rqstp, csum, rp)) - return rp; + ++entries; + if (nfsd_cache_match(rqstp, csum, rp)) { + ret = rp; + break; + } + } + + /* tally hash chain length stats */ + if (entries > longest_chain) { + longest_chain = entries; + longest_chain_cachesize = num_drc_entries; + } else if (entries == longest_chain) { + /* prefer to keep the smallest cachesize possible here */ + longest_chain_cachesize = min(longest_chain_cachesize, + num_drc_entries); } - return NULL; + + return ret; } /* @@ -573,6 +594,8 @@ static int nfsd_reply_cache_stats_show(struct seq_file *m, void *v) seq_printf(m, "cache misses: %u\n", nfsdstats.rcmisses); seq_printf(m, "not cached: %u\n", nfsdstats.rcnocache); seq_printf(m, "payload misses: %u\n", payload_misses); + seq_printf(m, "longest chain len: %u\n", longest_chain); + seq_printf(m, "cachesize at longest: %u\n", longest_chain_cachesize); spin_unlock(&cache_lock); return 0; } -- 2.20.1