Messages in this thread | | | Date | Thu, 19 May 2022 19:41:59 +0200 | Subject | Re: [PATCH v5 0/3] FUSE: Implement atomic lookup + open/create | From | Bernd Schubert <> |
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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
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