-----Original Message----- From: J. Bruce Fields [mailto:bfields@xxxxxxxxxxxx] Sent: Fri 6/6/2008 11:09 AM To: Weathers, Norman R. Cc: linux-nfs@xxxxxxxxxxxxxxx Subject: Re: Problems with large number of clients and reads On Tue, Jun 03, 2008 at 01:50:01PM -0500, Norman Weathers wrote: > Hello all, > > We are having some issues with some high throughput servers of ours. > > Here is the issue, we are using a vanilla 2.6.22.14 kernel on a node > with 2 Dual Core Intels (3 GHz) and 16 GB of ram. The files that are > being served are around 2 GB each, and there are usually 3 to 5 of them > being read, so once read they fit into memory nicely, and when all is > working correctly, we have a perfectly filled cache, with almost no disk > activity. > > When we have large NFS activity (say, 600 to 1200 clients) connecting to > the server(s), they can get into a state where they are using up all of > memory, but they are dropping cache. slabtop is showing 13 GB of memory > being used by the size-4096 slab object. We have two ethernet channels > bonded, so we see in excess of 240 MB/s of data flowing out of the box, > and all of the sudden, disk activity has risen to 185 MB/s. This > happens if we are using 8 or more nfs threads. If we limit the threads > to 6 or less, this doesn't happen. Of course, we are starving clients, > but at least the jobs that my customers are throwing out there are > progressing. The question becomes, what is causing the memory to be > used up by the slab size-4096 object? Why when all of the sudden a > bunch of clients ask for data does this object grow from 100 MB to 13 > GB? I have set the memory settings to something that I thought was > reasonable. > > Here is some more of the particulars: > > sysctl.conf tcp memory settings: > > # NFS Tuning Parameters > sunrpc.udp_slot_table_entries = 128 > sunrpc.tcp_slot_table_entries = 128 > vm.overcommit_ratio = 80 > > net.core.rmem_max=524288 > net.core.rmem_default=262144 > net.core.wmem_max=524288 > net.core.wmem_default=262144 > net.ipv4.tcp_rmem = 8192 262144 524288 > net.ipv4.tcp_wmem = 8192 262144 524288 > net.ipv4.tcp_sack=0 > net.ipv4.tcp_timestamps=0 > vm.min_free_kbytes=50000 > vm.overcommit_memory=1 > net.ipv4.tcp_reordering=127 > > # Enable tcp_low_latency > net.ipv4.tcp_low_latency=1 > > Here is a current reading from a slabtop of a system where this error is > happening: > > 3007154 3007154 100% 4.00K 3007154 1 12028616K size-4096 > > Note the size of the object cache, usually it is 50 - 100 MB (I have > another box with 32 threads and the same settings which is bouncing > between 50 and 128 MB right now). > > I have a lot of client boxes that need access to these servers, and > would really benefit from having more threads, but if I increase the > number of threads, it pushes everything out of cache, forcing re-reads, > and really slows down our jobs. > > Any thoughts on this? >I'd've thought that suggests a leak of memory allocated by kmalloc(). >Does the size-4096 cache decrease eventually, or does it stay that large >until you reboot? >--b. I would agree that it "looks" like a memory leak. If I restart NFS, the size-4096 cache goes from 12 GB to under 50 MB, but then depending upon how hard the box is utilized, it starts to climb back up. I have seen it climb back up to 3 or 4 GB right after the restart, but that is much better because the regular disk cache will grow from the 2 GB that it was pressured into back to 5 or 8 GB, so all of the files have been reread into memory and things are progressing smoothly. It is weird. I really think that this has to do with a lot of connections happening at once, because I can run slabtop and see a node that is running full out, but only have a couple hundred megs of the size-4096 slab being used, and then turn around and see another node that is pushing out 245 MB/s and all of the sudden using over 12 GB of the size-4096. It is very odd... If I lower the number of threads from a usable 64 to a low of 3 threads, I have less of a chance of the servers going haywire, to the point of being so loaded they may crash or you cannot contact them over the network (fortunately, I have serial on these boxes so that I can get on the nodes if they reach that point). If I run 8 threads, and with enough clients, I can bring down one of these servers. size-4096 goes through the roof, and depending on the hour of the day, the server can either crash or becomes unresponsive. Norman -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
