Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, May 18, 2010 at 8:55 PM, Corentin Chary
<corentin.chary@gmail.com> wrote:
> I believe that if the allocation size is large enougth, getting a
> zeroed page can be almost free with clever memory management.
> Could you try to re-run your test with calloc and see what it does ?
> Speeding up all mallocz calls is probably a good idea :)

I think that scenario is libc using mmap to grab zeroed pages from the
kernel.  If the kernel manages zeroed pages so that zeroing is
performed out-of-line (e.g. there is a pool of zeroed pages waiting),
then it appears that calloc() is getting zeroed memory cheaply.

I'll rerun with profiling tomorrow to see if calloc() makes a
difference for general qemu_mallocz() usage.

In the case of virtio-blk requests, we definitely shouldn't be
clearing that memory even if calloc() were cheaper.  Much of the
request structure will not be touched in an average I/O request.
Using zeroed pages is wasteful because they aren't needed here.

Stefan