2012/3/23 Myklebust, Trond <Trond.Myklebust@xxxxxxxxxx>: > On Fri, 2012-03-23 at 07:49 -0400, Jim Rees wrote: >> Vivek Trivedi wrote: >> >> 204800 bytes (200.0KB) copied, 0.027074 seconds, 7.2MB/s >> Read speed for 200KB file is 7.2 MB >> >> 104857600 bytes (100.0MB) copied, 9.351221 seconds, 10.7MB/s >> Read speed for 100MB file is 10.7 MB >> >> As you see read speed for 200KB file is only 7.2MB/sec while it is >> 10.7 MB/sec when we read 100MB file. >> Why there is so much difference in read performance ? >> Is there any way to achieve high read speed for small files ? >> >> That seems excellent to me. 204800 bytes at 11213252 per sec would be 18.2 >> msec, so your per-file overhead is around 9 msec. The disk latency alone >> would normally be more than that. > > ...and the reason why the performance is worse for the 200K file > compared to the 100M one is easily explained. > > When opening the file for reading, the client has a number of > synchronous RPC calls to make: it needs to look up the file, check > access permissions and possibly revalidate its cache. All these tasks > have to be done in series (you cannot do them in parallel), and so the > latency of each task is limited by the round-trip time to the server. > > On the other hand, once you get to doing READs, the client can send a > bunch of readahead requests in parallel, thus ensuring that the server > can use all the bandwidth available to the TCP connection. > > So your result is basically showing that for small files, the proportion > of (readahead) tasks that can be done in parallel is smaller. This is as > expected. > > -- > Trond Myklebust > Linux NFS client maintainer > > NetApp > Trond.Myklebust@xxxxxxxxxx > www.netapp.com > Dear Trond. I agree your answer. Thanks a lot for your specific explaination. -- 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
