On Thu, 7 Feb 2013 13:03:16 -0500 Jeff Layton <jlayton@xxxxxxxxxx> wrote: > On Thu, 7 Feb 2013 10:51:02 -0500 > Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > > > > > On Feb 7, 2013, at 9:51 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > > Now that we're allowing more DRC entries, it becomes a lot easier to hit > > > problems with XID collisions. In order to mitigate those, calculate the > > > crc32 of up to the first 256 bytes of each request coming in and store > > > that in the cache entry, along with the total length of the request. > > > > I'm happy to see a checksummed DRC finally become reality for the Linux NFS server. > > > > Have you measured the CPU utilization impact and CPU cache footprint of performing a CRC computation for every incoming RPC? I'm wondering if a simpler checksum might be just as useful but less costly to compute. > > > > No, I haven't, at least not in any sort of rigorous way. It's pretty > negligible on "normal" PC hardware, but I think most intel and amd cpus > have instructions for handling crc32. I'm ok with a different checksum, > we don't need anything cryptographically secure here. I simply chose > crc32 since it has an easily available API, and I figured it would be > fairly lightweight. > After an abortive attempt to measure this with ftrace, I ended up hacking together a patch to just measure the latency of the nfsd_cache_csum/_crc functions to get some rough numbers. On my x86_64 KVM guest, the avg time to calculate the crc32 is ~1750ns. Using IP checksums cuts that roughly in half to ~800ns. I'm not sure how best to measure the cache footprint however. Neither seems terribly significant, especially given the other inefficiencies in this code. OTOH, I guess those latencies can add up, and I don't see any need to use crc32 over the net/checksum.h routines. We probably ought to go with my RFC patch from yesterday. This could look very different on other arches too of course, but I don't have a test rig for any other arches at the moment. -- Jeff Layton <jlayton@xxxxxxxxxx> -- 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
