Re: Unexpected splice "always copy" behavior observed

Linus Torvalds
2010-05-19T19:35:10Z

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)
sync_file_range(fd, (index-1)*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER);
}

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.

Linus