Re: Listen backlog set to 64

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Dec 01, 2010 at 01:28:40PM -0500, Chuck Lever wrote:
> 
> On Dec 1, 2010, at 1:18 PM, Mark Hills wrote:
> 
> > On Tue, 30 Nov 2010, J. Bruce Fields wrote:
> > 
> >> On Tue, Nov 30, 2010 at 05:50:52PM +0000, Mark Hills wrote:
> > [...]
> >>> Our investigation brings us to rpc.mountd and mount.nfs communicating. In 
> >>> the client log we see messages like:
> >>> 
> >>>  Nov 24 12:09:43 nyrd001 automount[3782]: >> mount.nfs: mount to NFS server 'ss1a:/mnt/raid1/banana' failed: timed out, giving up
> >>> 
> >>> Using strace and isolating one of these, I can see a non-blocking connect 
> >>> has already managed to make a connection and even send/receive some data. 
> >>> 
> >>> But soon a timeout of 9999 milliseconds in poll() causes a problem in 
> >>> mount.nfs when waiting for a response of some sort. The socket in question 
> >>> is a connection to mountd:
> >>> 
> >>>  26512 futex(0x7ff76affa540, FUTEX_WAKE_PRIVATE, 1) = 0
> >>>  26512 write(3, "\200\0\0(j\212\254\365\0\0\0\0\0\0\0\2\0\1\206\245\0\0\0\3\0\0\0\0\0\0\0\0"..., 44) = 44
> >>>  26512 poll([{fd=3, events=POLLIN}], 1, 9999 <unfinished ...>
> >>> 
> >>> When it returns:
> >>> 
> >>>  26512 <... poll resumed> )              = 0 (Timeout)
> >>>  26512 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
> >>>  26512 rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], [], 8) = 0
> >>>  26512 close(3)                          = 0
> >>>  26512 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
> >>>  26512 write(2, "mount.nfs: mount to NFS server '"..., 100) = 100
> >>> 
> >>> There's no re-try from here, just a failed mount.
> >> 
> >> That does sound wrong.  I'm not at all familiar with automount,
> >> unfortunately; how is it invoking mount.nfs?
> >> 
> >>> What is the source of this 9999 millisecond timeout used by poll() in 
> >>> mount.nfs? It was not clear in an initial search of nfs-utils and glibc, 
> >>> but I need more time to investigate.
> >>> 
> >>> If the server is being too slow to respond, what could the cause of this 
> >>> be? Multiple threads are already in use, but it seems like they are not 
> >>> all in use because a thread is able to accept() the connection. I haven't 
> >>> been able to pin this on the forward/reverse DNS lookup used by 
> >>> authentication and logging.
> >> 
> >> Can you tell where the mountd threads are typically waiting?
> > 
> > Here's a trace from mountd. Note the long pause after fdatasync():
> > 
> > 31156 12:09:40 open("/var/lib/nfs/rmtab", O_WRONLY|O_CREAT|O_APPEND, 0666) = 14 <0.000010>
> > 31156 12:09:40 fstat(14, {st_mode=S_IFREG|0644, st_size=62642, ...}) = 0 <0.000005>
> > 31156 12:09:40 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2af00c929000 <0.000007>
> > 31156 12:09:40 fstat(14, {st_mode=S_IFREG|0644, st_size=62642, ...}) = 0 <0.000005>
> > 31156 12:09:40 lseek(14, 62642, SEEK_SET) = 62642 <0.000006>
> > 31156 12:09:40 write(14, "192.168.14.165:/mnt/raid1/abcde_"..., 57) = 57 <0.000013>
> > 31156 12:09:40 fdatasync(14)            = 0 <15.413401>
> > 31156 12:09:55 close(14)                = 0 <0.000008>
> > 31156 12:09:55 munmap(0x2af00c929000, 4096) = 0 <0.000083>
> > 31156 12:09:55 close(13)                = 0 <0.000008>
> > 
> > This pause exceeds the mysterious 9999 millisecond timeout allowed at the 
> > client.
> > 
> > fdatasync() is happening at every write of rmtab -- a lot. Occasionally it 
> > takes a long time. The syncing of the rmtab seems a reasonable explanation 
> > why more threads does not help -- all mountd threads lock up.
> > 
> > The strace I am analysing here contains 670 calls to fdatasync() in a 16 
> > minute period. All of them take < 1 second, except for this burst:
> > 
> > 31156 12:09:11 fdatasync(14)            = 0 <0.000010>
> > 31156 12:09:11 fdatasync(15)            = 0 <0.136285>
> > 31156 12:09:12 fdatasync(14)            = 0 <0.000010>
> > 31156 12:09:12 fdatasync(15)            = 0 <14.911323>
> > 31156 12:09:27 fdatasync(14)            = 0 <1.298896>
> > 31156 12:09:29 fdatasync(14)            = 0 <1.159262>
> > 31156 12:09:33 fdatasync(14)            = 0 <1.447169>
> > 31156 12:09:35 fdatasync(14)            = 0 <1.345562>
> > 31156 12:09:40 fdatasync(14)            = 0 <0.000010>
> > 31156 12:09:40 fdatasync(14)            = 0 <15.413401>
> > 31156 12:09:55 fdatasync(17)            = 0 <0.000010>
> > 31156 12:09:55 fdatasync(17)            = 0 <1.068503>
> > 31156 12:09:57 fdatasync(18)            = 0 <0.000010>
> > 31156 12:09:57 fdatasync(18)            = 0 <0.089682>
> > 
> > /var/lib/nfs/rmtab[.tmp] is on an ext3 filesystem, the system disk.
> > 
> > The server is only acting as a 'regular' and dedicated file server, but 
> > presumably some other process is causing /var to become erratic. Unless 
> > continual writing and syncing to a small (64KiB) file could cause this to 
> > happen periodically?
> 
> fdatasync() will write all outstanding dirty data on that partition to persistent storage, not just the dirty data for that file.  If /var is on a partition that has a lot of other activity, then this is the expected outcome, unfortunately.  I would expect such delays to be due to a large amount of outstanding writes, not to periodic synchronous writes.
> 
> This problem may be addressed in recent versions of nfs-utils.  I see a commit from Ben Myers on February 12 of this year that seems on point.  nfs-utils-1.2.2 has it, I think?

Doh, yes, looks like the code I quoted went into nfs-utils 1.2.2 with
Ben's commit.

--b.