On Mon, Jul 10 2017, Karel Zak wrote:
> On Mon, Jul 10, 2017 at 05:17:27PM +1000, NeilBrown wrote:
>> On Fri, Jul 07 2017, Phil Kauffman wrote:
>>
>> >> I can imagine /etc/mtab being read once for every line in /etc/exports,
>> >> but unless your /etc/exports is very big, I can't easily see why it
>> >> would be read 900 times.
>> >> Maybe lots of different mount points are being accessed by something and
>> >> each one triggers a few reads...
>> >>
>> >> Can you show your /etc/exports file?
>> >
>> > These get generated via puppet and dropped into /etc/exports.d/
>> ...
>> >
>> > Here we can see that there are 960 files in /etc/exports.d
>> > root@storage1:~# find /etc/exports.d/ -type f | wc
>> > 960 960 88701
>>
>> Ahhh.. 960 exports. That could do it.
>>
>> $ grep -c crossmnt all_etc_exports.d_in_one_file.txt
>> 957
>>
>> I get 957 - much the same number.
>>
>> $ grep open redacted_strace.txt | uniq -c
>> 2 open("/var/lib/nfs/etab", O_RDONLY) = 6
>> 942 open("/etc/mtab", O_RDONLY|O_CLOEXEC) = 6
>> 1 open("/proc/net/rpc/nfsd.export/channel", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 6
>>
>> /etc/mtab was ready 942 for a single access. I would have expected 957,
>> but maybe the system is dynamic and something changed between the two
>> samples.
>>
>> This makes it fairly clear what is happening. Now we just need to fix
>> it.
>> One option would be to cache some of the details extracted from
>> /etc/mtab, but that could get messy.
>
> Would be possible to read /proc/self/mounts only once and use poll()
> to detect changes?
Sure it is possible. All we need is code....
>
> The issue is not only the open()+read() in the process, but also
> kernel has to compose the file from the mount tree. It's expensive.
That certainly contributes to the problem. If we cache the content, we
can process it fast. If we avoid wanting to look at the content, it
doesn't matter how fast processes it is (because we don't).
So yes, that might be part of the solution. But not the whole solution.
Thanks,
NeilBrown
>
> Karel
>
>
> --
> Karel Zak <kzak@xxxxxxxxxx>
> http://karelzak.blogspot.com
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