From: Bernd Schubert <bschubert@ddn.com>
To: Miklos Szeredi <miklos@szeredi.hu>,
Dharmendra Singh <dharamhans87@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>,
linux-fsdevel@vger.kernel.org,
fuse-devel <fuse-devel@lists.sourceforge.net>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 0/3] FUSE: Implement atomic lookup + open/create
Date: Thu, 19 May 2022 19:41:59 +0200 [thread overview]
Message-ID: <f3555e3c-06d9-4d19-d3a2-9a2779937e83@ddn.com> (raw)
In-Reply-To: <CAJfpegsDxsMsyfP4a_5H1q91xFtwcEdu9-WBnzWKwjUSrPNdmw@mail.gmail.com>
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
next prev parent reply other threads:[~2022-05-19 17:42 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-17 10:07 [PATCH v5 0/3] FUSE: Implement atomic lookup + open/create Dharmendra Singh
2022-05-17 10:07 ` [PATCH v5 1/3] FUSE: Avoid lookups in fuse create Dharmendra Singh
2022-05-17 21:21 ` Vivek Goyal
2022-05-18 17:41 ` Vivek Goyal
2022-05-18 17:44 ` Vivek Goyal
2022-05-18 20:28 ` Bernd Schubert
2022-05-17 10:07 ` [PATCH v5 2/3] FUSE: Rename fuse_create_open() to fuse_atomic_common() Dharmendra Singh
2022-05-17 10:07 ` [PATCH v5 3/3] FUSE: Implement atomic lookup + open Dharmendra Singh
2022-05-19 9:39 ` [PATCH v5 0/3] FUSE: Implement atomic lookup + open/create Miklos Szeredi
2022-05-19 13:13 ` Miklos Szeredi
2022-05-19 17:41 ` Bernd Schubert [this message]
2022-05-19 18:16 ` Miklos Szeredi
2022-05-19 20:47 ` [fuse-devel] " Bernd Schubert
2022-05-19 19:33 ` Vivek Goyal
2023-06-01 11:16 ` Bernd Schubert
2023-06-01 11:50 ` Miklos Szeredi
2023-06-01 12:01 ` Bernd Schubert
2023-06-01 12:18 ` Miklos Szeredi
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=f3555e3c-06d9-4d19-d3a2-9a2779937e83@ddn.com \
--to=bschubert@ddn.com \
--cc=dharamhans87@gmail.com \
--cc=fuse-devel@lists.sourceforge.net \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=miklos@szeredi.hu \
--cc=vgoyal@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).