Re: [fuse-devel] [fuse] Unexpectedly large number of getattr() and lookup requests

[Nikolaus Rath <Nikolaus@rath.org>]

On Dec 04 2018, Miklos Szeredi <miklos@szeredi.hu> wrote:
> On Sat, Dec 1, 2018 at 11:00 AM Nikolaus Rath <Nikolaus@rath.org> wrote:
>>
>> On Nov 30 2018, Miklos Szeredi <miklos@szeredi.hu> wrote:
>> > On Thu, Nov 29, 2018 at 10:20 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
>> >>
>> >> Hello,
>> >>
>> >> I am seeing an unexpectedly large number of getattr() and lookup()
>> >> requests being sent to userspace fuse. I am setting a very large
>> >> attr_timeout and entry_timeout, so I would have expected that the
>> >> maximum number of getattr() and lookup() requests is capped by the
>> >> number of distinct files in the filesystem plus the number of forget
>> >> requests.
>> >>
>> >> However, actual numbers are much higher. For example, when running tests
>> >> on a filesystem with 2960 directory entries, I am getting scenarios
>> >> with 203447 lookup requests, 12970 getattr requests, and zero forget
>> >> requests.
>> >>
>> >> Did I misunderstand something about how dentry and attribute caching
>> >> works?
>> >
>> > Debug log might be useful.
>>
>> Here you go!
>>
>> https://www.dropbox.com/s/tg4vyshz4g18sub/fuse-debug.log.xz?dl=1
>
> $ grep LOOKUP fuse-debug.log | wc -l
> 20786
> $ grep -A2 LOOKUP fuse-debug.log |grep "No such file or directory"  | wc -l
> 20116
>
> Since it's a lowlevel log, we can't see what the negative lookups are
> for, but by returning ENOENT it is guaranteed that the negative lookup
> can't be cached.  Calling fuse_reply_entry() instead with a zero
> nodeid the negative entry can also be cached, which probably helps to
> reduce the number of lookups.

Uh, right. I forgot about that. Thanks!

> The GETATTR requests are due atime invalidations.

Could you elaborate? I'm not sure what that means here. What is an atime
invalidation?


Best,
-Nikolaus

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