* David Ahern <dsah...@gmail.com> wrote:
> On 10/15/13 10:06 AM, Ingo Molnar wrote:
> > splice() is very fast and should be able to process a lot of pages in
> > one go, so the feedback loop should be pretty weak. mmap() triggers
> > kernel code as well, every time we run out of the 64 MB window we got
> > to remap it, right?
>
>
> Yes, 1 mmap, 1 munmap for every 64MB. Compare to the write() case which
> calls write() for each mmap each time through the mmap_read loop.
>
> I am conjecturing that splice would follow the write model in the sense
> of a ring buffer has N bytes, call splice to copy the data from the ring
> buffer to the file. So, splice saves on the memcpy, but not the
> syscalls.
Well, splice can be 64 MB granular as well - but it depends on how
buffered the pipes are (splice always uses pipes to shuffle around pages).
It's possible to have pipes with larger buffering - see F_SETPIPE_SZ. The
current limit (for unprivileged users?) is:
fs/pipe.c:unsigned int pipe_max_size = 1048576;
[But I haven't actually used this to increase splice() size yet.]
Thanks,
Ingo
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