Re: Unexpected splice "always copy" behavior observed

From: Mathieu Desnoyers
Date: Wed May 19 2010 - 17:49:17 EST

* Linus Torvalds (torvalds@xxxxxxxxxxxxxxxxxxxx) wrote:
> On Wed, 19 May 2010, Mathieu Desnoyers wrote:
> >
> > Good point. This discard flag might do the trick and let us keep things simple.
> > The major concern here is to keep the page cache disturbance relatively low.
> > Which of new page allocation or stealing back the page has the lowest overhead
> > would have to be determined with benchmarks.
> We could probably make it easier somehow to do the writeback and discard
> thing, but I have had _very_ good experiences with even a rather trivial
> file writer that basically used (iirc) 8MB windows, and the logic was very
> trivial:
> - before writing a new 8M window, do "start writeback"
> (SYNC_FILE_RANGE_WRITE) on the previous window, and do
> a wait (SYNC_FILE_RANGE_WAIT_AFTER) on the window before that.
> in fact, in its simplest form, you can do it like this (this is from my
> "overwrite disk images" program that I use on old disks):
> for (index = 0; index < max_index ;index++) {
> if (write(fd, buffer, BUFSIZE) != BUFSIZE)
> break;
> /* This won't block, but will start writeout asynchronously */
> sync_file_range(fd, index*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WRITE);
> /* This does a blocking write-and-wait on any old ranges */
> if (index)
> }
> and even if you don't actually do a discard (maybe we should add a
> SYNC_FILE_RANGE_DISCARD bit, right now you'd need to do a separate
> fadvise(FADV_DONTNEED) to throw it out) the system behavior is pretty
> nice, because the heavy writer gets good IO performance _and_ leaves only
> easy-to-free pages around after itself.

Great! I just implemented it in LTTng and it works very well !

A faced a small counter-intuitive fadvise behavior though.

posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED);

only seems to affect the parts of a file that already exist. So after each
splice() that appends to the file, I have to call fadvise again. I would have
expected the "0" len parameter to tell the kernel to apply the hint to the whole
file, even parts that will be added in the future. I expect we have this
behavior because fadvise() was initially made with read behavior in mind rather
than write.

For the records, I do a fadvice+async range write after each splice(). Also,
after each subbuffer write, I do a blocking write-and-wait on all pages that are
in the subbuffer prior to the one that has just been written, instead of using
the fixed 8MB window.



Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
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