Re: [RFC PATCH 5/5] fuse: add documentation of sysfs parameter tolimit maximum fuse request size

[HAYASAKA Mitsuo]

Hi Rob,

Thank you for your comments.


(2012/07/06 21:54), Rob Landley wrote:
> On 07/05/2012 05:51 AM, Mitsuo Hayasaka wrote:
> > Add an explantion about the sysfs parameter to the limit
> > maximum read/write request size.
> >
> > Signed-off-by: Mitsuo Hayasaka<mitsuo.hayasaka.hu@xxxxxxxxxxx>
> > Cc: Rob Landley<rob@xxxxxxxxxxx>
> > Cc: Miklos Szeredi<miklos@xxxxxxxxxx>
> > ---
> >
> >   Documentation/filesystems/fuse.txt |   17 ++++++++++++++++-
> >   1 files changed, 16 insertions(+), 1 deletions(-)
> >
> > diff --git a/Documentation/filesystems/fuse.txt b/Documentation/filesystems/fuse.txt
> > index 13af4a4..e6ffba3 100644
> > --- a/Documentation/filesystems/fuse.txt
> > +++ b/Documentation/filesystems/fuse.txt
> > @@ -108,13 +108,28 @@ Mount options
> >
> >     With this option the maximum size of read operations can be set.
> >     The default is infinite.  Note that the size of read requests is
> > -  limited anyway to 32 pages (which is 128kbyte on i386).
> > +  limited to 32 pages (which is 128kbyte on i386) if direct_io
> > +  option is not specified. When direct_io option is specified,
> > +  the request size is limited to max_pages_per_req sysfs parameter.
>
> "Note that the maximum size of read requests defaults to 32 pages (128k
> on i386), use max_pages_per_req to change this default."
>
> And then describe max_page_per_req sufficiently thoroughly below, all in
> one place.

OK, I will revise it.

> (By the way, has anybody actually tested it with a single page as the
> limit? Does that work?)


This patch series enables the maximum request size to change to arbitrary
number from 32 to 256, and cannot set it to less than 32 pages.

> >   'blksize=N'
> >
> >     Set the block size for the filesystem.  The default is 512.  This
> >     option is only valid for 'fuseblk' type mounts.
> >
> > +Sysfs parameter
> > +~~~~~~~~~~~~~~~
> > +
> > +The sysfs parameter max_pages_per_req limits the maximum page size per
> > +FUSE request.
>
> No, it limits the maximum size of a data request and the units are
> decimal number of pages. It doesn't change the size of memory pages in
> the system.

You are right. I will revise it.


> Also, your first hunk implies this setting only takes effect if they
> mounted with "-o direct_io", is that true?


The request size increases using max_pages_per_req for read operation w/
direct_io, and write operation w/ and w/o direct_io. But it is not changed
for read operation w/o direct_io. So, it is true if only focusing on read
operation.


> > +	/sys/fs/fuse/max_pages_per_req
> > +
> > +The default is 32 pages. It can be changed from 32 to 256 pages, which
> > +may improve the read/write throughput optimizing it. This change is
> > +effective per mount. Therefore, the re-mounting of FUSE filesystem
> > +is required after changing it.
>
> I'd say "Changing it to 256 pages may improve read/write throguhput on
> systems with enough memory. Existing FUSE mounts must be remounted for
> this change to take effect."
>
> I.E. don't imply 32 and 256 are the only options unless they are. (Is
> there some requirement that it be a power of 2, or just a good idea?)


Here, I wanted to imply that the max_paegs_per_req can be changed to
arbitrary number from 32 to 256. I will revise it since this explanation
is misleading.

Also, there is no requirement that it be a power of 2 although it is a
good idea if only focusing on kmalloc(). One of the reasons to introduce
the max_pages_per_req sysfs parameter is to let the libfuse get the
current maximum request size and change the MIN_BUFSIZE limitation
according to it to avoid an waste of memory in userspace.


> And per-mount sounds like you're setting it for a specific mount point,
> so if I have three mounts there would be three entries under
> /sys/fs/fuse, which does not seem to be the case. (Which is odd, because
> you'd think there would be an "-o max_pages_per_req=128" that _would_
> set this per-mount if the value actually used is cached in the
> superblock, but I'm not seeing one...)


The max_pages_per_req is a system limitation controlled by the administrator.
The actual number of allocated pages per request can be changed using
max_read/max_write mount options below this system limitation.

I will revise and resubmit this patch series soon.

Thanks,

> Rob
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