Re: rpc.mountd high cpu usage

Sander Klein
2013-12-23T11:07:53Z

On , J. Bruce Fields wrote:
> On Fri, Dec 13, 2013 at 08:32:56PM +0100, Sander Klein wrote:
>> On , J. Bruce Fields wrote:
>> >>>I think what happens is that exportfs flushes the kernel's export cache
>> >>>at which point every use of an uncached export triggers an upcall to
>> >>>mountd. That upcall is probably visible in the strace as a read of a
>> >>>file descriptor associated with /proc/net/sunrpc/nfsd.fh/content.
>> >>>
>> >>>That upcall is handled by nfs-utils/utils/mountd/cache.c:nfsd_fh(),
>> >>>which is given a filehandle fragment identifying the filesystem in
>> >>>question and has to match it to an export.
>> >>>
>> >>>That's done by match_fsid(). Which does do a stat of the export path,
>> >>>but not of all the devices.... That's probably happening in one of the
>> >>>libblkid calls in uuid_by_path()? I wonder if there's something wrong
>> >>>with libblkid configuration or with the way we're using it?
>> >>
>> >>Is there any way I can help getting this fixed? My coding skills are
>> >>limited but I am very willing to help in any way I can.
>> >
>> >I wonder if ltrace could help determine if libblkid is where most of
>> >those stat's are coming from (and if so, which calls)?
>> >
>> >May also be worth reading up on libblkid (man libblkid, etc.) and
>> >checking your configuration to make sure there's nothing obvious broken
>> >there. (If so, maybe the libblkid commandline tools would exhibit the
>> >same problem?)
>>
>> I didn't have any config in /etc/blkid.conf and the manpage says it
>> will use EVALUATE=udev,scan which in fact scans through all the
>> /dev/disk/by-* dirs and parses the /proc/partitions. I put
>> 'EVALUATE=scan' in /etc/blkid.conf but that doesn't help. I
>> restarted anything NFS related...
>>
>> I also attached ltrace to the rpc.mountd process. The output of the
>> trace is at http://pastebin.com/ika1Vetq . I'm not sure what I'm
>> looking for. The ltrace is done on a server with only 10 harddisks
>> but the strace showed the same behavour of newfstatat-ing every disk
>> in a lot of ways.
>
> Hm. I *think* that those SYS_262's are the newfstatat's. And it looks
> like they happen as part of the implementation of
> blkid_devno_to_devname
> (just because you see that call start above the SYS_262's and return
> only after).
>
> That is indeed called from utils/mountd/cache.c:get_uuid_blkdev().
>
> So libblkid is mapping device numbers to paths by stat'ing lots of
> stuff
> under /dev.
>
> I'm not sure what to do about that.... Cc'ing util-linux in case they
> can help.
>
> --b.

Anything else I can do to help a bit getting this fixed?

Regards,

Sander