lkml.org 
[lkml]   [2022]   [May]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v5 0/3] FUSE: Implement atomic lookup + open/create
From


On 5/19/22 11:39, Miklos Szeredi wrote:
> On Tue, 17 May 2022 at 12:08, Dharmendra Singh <dharamhans87@gmail.com> wrote:
>>
>> In FUSE, as of now, uncached lookups are expensive over the wire.
>> E.g additional latencies and stressing (meta data) servers from
>> thousands of clients. These lookup calls possibly can be avoided
>> in some cases. Incoming three patches address this issue.
>>
>>
>> Fist patch handles the case where we are creating a file with O_CREAT.
>> Before we go for file creation, we do a lookup on the file which is most
>> likely non-existent. After this lookup is done, we again go into libfuse
>> to create file. Such lookups where file is most likely non-existent, can
>> be avoided.
>
> I'd really like to see a bit wider picture...
>
> We have several cases, first of all let's look at plain O_CREAT
> without O_EXCL (assume that there were no changes since the last
> lookup for simplicity):
>
> [not cached, negative]
> ->atomic_open()
> LOOKUP
> CREATE
>
> [not cached, positive]
> ->atomic_open()
> LOOKUP
> ->open()
> OPEN
>
> [cached, negative, validity timeout not expired]
> ->d_revalidate()
> return 1
> ->atomic_open()
> CREATE
>
> [cached, negative, validity timeout expired]
> ->d_revalidate()
> return 0
> ->atomic_open()
> LOOKUP
> CREATE
>
> [cached, positive, validity timeout not expired]
> ->d_revalidate()
> return 1
> ->open()
> OPEN
>
> [cached, positive, validity timeout expired]
> ->d_revalidate()
> LOOKUP
> return 1
> ->open()
> OPEN
>
> (Caveat emptor: I'm just looking at the code and haven't actually
> tested what happens.)
>
> Apparently in all of these cases we are doing at least one request, so
> it would make sense to make them uniform:
>
> [not cached]
> ->atomic_open()
> CREATE_EXT
>
> [cached]
> ->d_revalidate()
> return 0
> ->atomic_open()
> CREATE_EXT
>
> Similarly we can look at the current O_CREAT | O_EXCL cases:
>
> [not cached, negative]
> ->atomic_open()
> LOOKUP
> CREATE
>
> [not cached, positive]
> ->atomic_open()
> LOOKUP
> return -EEXIST
>
> [cached, negative]
> ->d_revalidate()
> return 0 (see LOOKUP_EXCL check)
> ->atomic_open()
> LOOKUP
> CREATE
>
> [cached, positive]
> ->d_revalidate()
> LOOKUP
> return 1
> return -EEXIST
>
> Again we are doing at least one request, so we can unconditionally
> replace them with CREATE_EXT like the non-O_EXCL case.
>
>
>>
>> Second patch handles the case where we open first time a file/dir
>> but do a lookup first on it. After lookup is performed we make another
>> call into libfuse to open the file. Now these two separate calls into
>> libfuse can be combined and performed as a single call into libfuse.
>
> And here's my analysis:
>
> [not cached, negative]
> ->lookup()
> LOOKUP
> return -ENOENT
>
> [not cached, positive]
> ->lookup()
> LOOKUP
> ->open()
> OPEN
>
> [cached, negative, validity timeout not expired]
> ->d_revalidate()
> return 1
> return -ENOENT
>
> [cached, negative, validity timeout expired]
> ->d_revalidate()
> return 0
> ->atomic_open()
> LOOKUP
> return -ENOENT
>
> [cached, positive, validity timeout not expired]
> ->d_revalidate()
> return 1
> ->open()
> OPEN
>
> [cached, positive, validity timeout expired]
> ->d_revalidate()
> LOOKUP
> return 1
> ->open()
> OPEN
>
> There's one case were no request is sent: a valid cached negative
> dentry. Possibly we can also make this uniform, e.g.:
>
> [not cached]
> ->atomic_open()
> OPEN_ATOMIC
>
> [cached, negative, validity timeout not expired]
> ->d_revalidate()
> return 1
> return -ENOENT
>
> [cached, negative, validity timeout expired]
> ->d_revalidate()
> return 0
> ->atomic_open()
> OPEN_ATOMIC
>
> [cached, positive]
> ->d_revalidate()
> return 0
> ->atomic_open()
> OPEN_ATOMIC
>
> It may even make the code simpler to clearly separate the cases where
> the atomic variants are supported and when not. I'd also consider
> merging CREATE_EXT into OPEN_ATOMIC, since a filesystem implementing
> one will highly likely want to implement the other as well.


Can you help me a bit to understand what we should change? I had also
already thought to merge CREATE_EXT and OPEN_ATOMIC - so agreed.
Shall we make the other cases more visible?

Also thanks a lot for you revalidate patch.


Thanks,
Bernd

Thanks,
Bernd

\
 
 \ /
  Last update: 2022-05-19 19:44    [W:0.154 / U:0.560 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site