misc/145189: nfsd performs abysmally under load

[Bruce Evans]

The following reply was made to PR misc/145189; it has been noted by GNATS.

From: Bruce Evans <brde at optusnet.com.au>
To: Rich Ercolani <rercola at acm.jhu.edu>
Cc: freebsd-gnats-submit at freebsd.org, freebsd-bugs at freebsd.org
Subject: Re: misc/145189: nfsd performs abysmally under load 
Date: Wed, 31 Mar 2010 02:50:16 +1100 (EST)

 On Tue, 30 Mar 2010, Rich Ercolani wrote:
 
 >> Description:
 > nfsd performs abysmally on this machine under conditions in which Solaris's NFS implementation is reasonably fast, and while local IO to the same filesystems is still zippy.
 
 Please don't format lines for 200+ column terminals.
 
 Does it work better when limited to 1 thread (nfsd -n 1)?  In at least
 some versions of it (or maybe in nfsiod), multiple threads fight each other
 under load.
 
 > For instance, copying a 4GB file over NFSv3 from a ZFS filesystem with the following flags [rw,nosuid,hard,intr,nofsc,tcp,vers=3,rsize=8192,wsize=8192,sloppy,addr=X.X.X.X](Linux client, the above is the server), I achieve 2 MB/s, fluctuating between 1 and 3. (pv reports 2.23 MB/s avg)
 >
 > Locally, on the server, I achieve 110-140 MB/s (at the end of pv, it reports 123 MB/s avg).
 >
 > I'd assume network latency, but nc with no flags other than port achieves 30-50 MB/s between server and client.
 >
 > Latency is also abysmal - ls on a randomly chosen homedir full of files, according to time, takes:
 > real    0m15.634s
 > user    0m0.012s
 > sys     0m0.097s
 > while on the local machine:
 > real	0m0.266s
 > user	0m0.007s
 > sys	0m0.000s
 
 It probably is latency.  nfs is very latency-sensitive when there are lots
 of small files.  Transfers of large files shouldn't be affected so much.
 
 > The server in question is a 3GHz Core 2 Duo, running FreeBSD RELENG_8. The kernel conf, DTRACE_POLL, is just the stock AMD64 kernel with all of the DTRACE-related options turned on, as well as the option to enable polling in the NIC drivers, since we were wondering if that would improve our performance.
 
 Enabling polling is a good way to destroy latency.  A ping latency of
 more that about 50uS causes noticable loss of performance for nfs, but
 LAN latency is usually a few times higher than that, and polling without
 increasing the clock interrupt frequency to an excessively high value
 gives a latency of at least 20 times higher than that.  Also, -current
 with debugging options is so bloated that even localhost has a ping
 latency of about 50uS on a Core2 (up from 2uS for FreeBSD-4 on an
 AthlonXP).  Anyway try nfs on localhost to see if reducing the latency
 helps.
 
 > We tested this with a UFS directory as well, because we were curious if this was an NFS/ZFS interaction - we still got 1-2 MB/s read speed and horrible latency while achieving fast throughput and latency local to the server, so we're reasonably certain it's not "just" ZFS, if there is indeed any interaction there.
 
 After various tuning and bug fixing (now partly committed by others) I get
 improvements like the following on low-end systems with ffs (I don't use
 zfs):
 - very low end with 100Mbps ethernet: little change; bulk transfers always
    went at near wire speed (about 10 MB/S)
 - low end with 1Gbps/S: bulk transfers up from 20MB/S to 45MB/S (local ffs
    50MB/S).  buildworld over nfs of 5.2 world down from 1200 seconds to 800
    seconds (this one is very latency-sensitive.  Takes about 750 seconds on
    local ffs).
 
 > Read speed of a randomly generated 6500 MB file on UFS over NFSv3 with the same flags as above: 1-3 MB/s, averaging 2.11 MB/s
 > Read speed of the same file, local to the server: consistently between 40-60 MB/s, averaging 61.8 MB/s [it got faster over time - presumably UFS was aggressively caching the file, or something?]
 
 You should use a file size larger than the size of main memory to prevent
 caching, especially for reads.  That is 1GB on my low-end systems.
 
 > Read speed of the same file over NFS again, after the local test:
 > Amusingly, worse (768 KB/s-2.2 MB/s, with random stalls - average reported 270 KB/s(!)).
 
 The random stalls are typical of the problem with the nfsd's getting
 in each other's way, and/or of related problems.  The stalls that I
 saw were very easy to see in real time using "netstat -I <interface>
 1" -- they happened every few seconds and lasted a second or 2.  But
 they were never long enough to reduce the throughput by more than a
 factor of 3, so I always got over 19 MB/S.  The throughput was reduced
 by approximately the ratio of stalled time to non-stalled time.
 
 >> How-To-Repeat:
 > 1) Mount multiple NFS filesystems from the server
 > 2) Watch as your operations latency and throughput rapidly sink to near-zero
 
 Multiple active nfs mounts are probably a different problem.  You certainly
 need more than 1 nfsd and/or nfsiod to handle them, and the stalls might
 be a result of not having enough daemons.
 
 Bruce