From 59a252ff8c0f2fa32c896f69d56ae33e641ce7ad Mon Sep 17 00:00:00 2001 From: Greg Banks <gnb@sgi.com> Date: Tue, 13 Jan 2009 21:26:35 +1100 Subject: knfsd: avoid overloading the CPU scheduler with enormous load averages Avoid overloading the CPU scheduler with enormous load averages when handling high call-rate NFS loads. When the knfsd bottom half is made aware of an incoming call by the socket layer, it tries to choose an nfsd thread and wake it up. As long as there are idle threads, one will be woken up. If there are lot of nfsd threads (a sensible configuration when the server is disk-bound or is running an HSM), there will be many more nfsd threads than CPUs to run them. Under a high call-rate low service-time workload, the result is that almost every nfsd is runnable, but only a handful are actually able to run. This situation causes two significant problems: 1. The CPU scheduler takes over 10% of each CPU, which is robbing the nfsd threads of valuable CPU time. 2. At a high enough load, the nfsd threads starve userspace threads of CPU time, to the point where daemons like portmap and rpc.mountd do not schedule for tens of seconds at a time. Clients attempting to mount an NFS filesystem timeout at the very first step (opening a TCP connection to portmap) because portmap cannot wake up from select() and call accept() in time. Disclaimer: these effects were observed on a SLES9 kernel, modern kernels' schedulers may behave more gracefully. The solution is simple: keep in each svc_pool a counter of the number of threads which have been woken but have not yet run, and do not wake any more if that count reaches an arbitrary small threshold. Testing was on a 4 CPU 4 NIC Altix using 4 IRIX clients, each with 16 synthetic client threads simulating an rsync (i.e. recursive directory listing) workload reading from an i386 RH9 install image (161480 regular files in 10841 directories) on the server. That tree is small enough to fill in the server's RAM so no disk traffic was involved. This setup gives a sustained call rate in excess of 60000 calls/sec before being CPU-bound on the server. The server was running 128 nfsds. Profiling showed schedule() taking 6.7% of every CPU, and __wake_up() taking 5.2%. This patch drops those contributions to 3.0% and 2.2%. Load average was over 120 before the patch, and 20.9 after. This patch is a forward-ported version of knfsd-avoid-nfsd-overload which has been shipping in the SGI "Enhanced NFS" product since 2006. It has been posted before: http://article.gmane.org/gmane.linux.nfs/10374 Signed-off-by: Greg Banks <gnb@sgi.com> Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu> --- net/sunrpc/svc_xprt.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) (limited to 'net') diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c index e588df5d6b34..0551b6b6cf8c 100644 --- a/net/sunrpc/svc_xprt.c +++ b/net/sunrpc/svc_xprt.c @@ -14,6 +14,8 @@ #define RPCDBG_FACILITY RPCDBG_SVCXPRT +#define SVC_MAX_WAKING 5 + static struct svc_deferred_req *svc_deferred_dequeue(struct svc_xprt *xprt); static int svc_deferred_recv(struct svc_rqst *rqstp); static struct cache_deferred_req *svc_defer(struct cache_req *req); @@ -298,6 +300,7 @@ void svc_xprt_enqueue(struct svc_xprt *xprt) struct svc_pool *pool; struct svc_rqst *rqstp; int cpu; + int thread_avail; if (!(xprt->xpt_flags & ((1<<XPT_CONN)|(1<<XPT_DATA)|(1<<XPT_CLOSE)|(1<<XPT_DEFERRED)))) @@ -309,12 +312,6 @@ void svc_xprt_enqueue(struct svc_xprt *xprt) spin_lock_bh(&pool->sp_lock); - if (!list_empty(&pool->sp_threads) && - !list_empty(&pool->sp_sockets)) - printk(KERN_ERR - "svc_xprt_enqueue: " - "threads and transports both waiting??\n"); - if (test_bit(XPT_DEAD, &xprt->xpt_flags)) { /* Don't enqueue dead transports */ dprintk("svc: transport %p is dead, not enqueued\n", xprt); @@ -353,7 +350,14 @@ void svc_xprt_enqueue(struct svc_xprt *xprt) } process: - if (!list_empty(&pool->sp_threads)) { + /* Work out whether threads are available */ + thread_avail = !list_empty(&pool->sp_threads); /* threads are asleep */ + if (pool->sp_nwaking >= SVC_MAX_WAKING) { + /* too many threads are runnable and trying to wake up */ + thread_avail = 0; + } + + if (thread_avail) { rqstp = list_entry(pool->sp_threads.next, struct svc_rqst, rq_list); @@ -368,6 +372,8 @@ void svc_xprt_enqueue(struct svc_xprt *xprt) svc_xprt_get(xprt); rqstp->rq_reserved = serv->sv_max_mesg; atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved); + rqstp->rq_waking = 1; + pool->sp_nwaking++; BUG_ON(xprt->xpt_pool != pool); wake_up(&rqstp->rq_wait); } else { @@ -633,6 +639,11 @@ int svc_recv(struct svc_rqst *rqstp, long timeout) return -EINTR; spin_lock_bh(&pool->sp_lock); + if (rqstp->rq_waking) { + rqstp->rq_waking = 0; + pool->sp_nwaking--; + BUG_ON(pool->sp_nwaking < 0); + } xprt = svc_xprt_dequeue(pool); if (xprt) { rqstp->rq_xprt = xprt; -- cgit v1.2.3