lkml.org 
[lkml]   [2013]   [Sep]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRE: [RFC] extending splice for copy offloading
    Date
    > -----Original Message-----
    > From: Ric Wheeler [mailto:rwheeler@redhat.com]
    > Sent: Monday, September 30, 2013 10:29 AM
    > To: Miklos Szeredi
    > Cc: J. Bruce Fields; Myklebust, Trond; Zach Brown; Anna Schumaker; Kernel
    > Mailing List; Linux-Fsdevel; linux-nfs@vger.kernel.org; Schumaker, Bryan;
    > Martin K. Petersen; Jens Axboe; Mark Fasheh; Joel Becker; Eric Wong
    > Subject: Re: [RFC] extending splice for copy offloading
    >
    > On 09/30/2013 10:24 AM, Miklos Szeredi wrote:
    > > On Mon, Sep 30, 2013 at 4:52 PM, Ric Wheeler <rwheeler@redhat.com>
    > wrote:
    > >> On 09/30/2013 10:51 AM, Miklos Szeredi wrote:
    > >>> On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields
    > >>> <bfields@fieldses.org>
    > >>> wrote:
    > >>>>> My other worry is about interruptibility/restartability. Ideas?
    > >>>>>
    > >>>>> What happens on splice(from, to, 4G) and it's a non-reflink copy?
    > >>>>> Can the page cache copy be made restartable? Or should splice() be
    > >>>>> allowed to return a short count? What happens on (non-reflink)
    > >>>>> remote copies and huge request sizes?
    > >>>> If I were writing an application that required copies to be
    > >>>> restartable, I'd probably use the largest possible range in the
    > >>>> reflink case but break the copy into smaller chunks in the splice case.
    > >>>>
    > >>> The app really doesn't want to care about that. And it doesn't want
    > >>> to care about restartability, etc.. It's something the *kernel* has
    > >>> to care about. You just can't have uninterruptible syscalls that
    > >>> sleep for a "long" time, otherwise first you'll just have annoyed
    > >>> users pressing ^C in vain; then, if the sleep is even longer,
    > >>> warnings about task sleeping too long.
    > >>>
    > >>> One idea is letting splice() return a short count, and so the app
    > >>> can safely issue SIZE_MAX requests and the kernel can decide if it
    > >>> can copy the whole file in one go or if it wants to do it in smaller
    > >>> chunks.
    > >>>
    > >> You cannot rely on a short count. That implies that an offloaded copy
    > >> starts at byte 0 and the short count first bytes are all valid.
    > > Huh?
    > >
    > > - app calls splice(from, 0, to, 0, SIZE_MAX)
    > > 1) VFS calls ->direct_splice(from, 0, to, 0, SIZE_MAX)
    > > 1.a) fs reflinks the whole file in a jiffy and returns the size of the file
    > > 1 b) fs does copy offload of, say, 64MB and returns 64M
    > > 2) VFS does page copy of, say, 1MB and returns 1MB
    > > - app calls splice(from, X, to, X, SIZE_MAX) where X is the new offset
    > > ...
    > >
    > > The point is: the app is always doing the same (incrementing offset
    > > with the return value from splice) and the kernel can decide what is
    > > the best size it can service within a single uninterruptible syscall.
    > >
    > > Wouldn't that work?
    > >
    > > Thanks,
    > > Miklos
    >
    > No.
    >
    > Keep in mind that the offload operation in (1) might fail partially. The target
    > file (the copy) is allocated, the question is what ranges have valid data.
    >
    > I don't see that (2) is interesting or really needed to be done in the kernel.
    > If nothing else, it tends to confuse the discussion....
    >

    Anna's figures, that were presented at Plumber's, show that (2) is still worth doing on the _server_ for the case of NFS.

    Cheers
    Trond
    \
     
     \ /
      Last update: 2013-09-30 23:21    [W:2.587 / U:0.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site