Re: [Qemu-devel] [PATCH] block/io: optimize bdrv_co_pwritev for small requests

[Kevin Wolf]

Am 2.05.2016 um 16:07 hat Peter Lieven geschrieben:
> Am 24.05.2016 um 15:59 schrieb Paolo Bonzini:
> >
> >On 24/05/2016 15:39, Peter Lieven wrote:
> >>          bytes += offset & (align - 1);
> >>          offset = offset & ~(align - 1);
> >Because the low bits have been masked away from offset and added to bytes,
> >
> >>+
> >>+        /* if head and tail fall into the same alignment
> >>+         * we can omit the second read as it would read
> >>+         * the same block again */
> >>+        if ((offset + bytes) & (align - 1) &&
> >... the first part is just "bytes & (align - 1)"...
> >
> >>+            offset / align == (offset + bytes) / align) {
> >... and the second part is just "bytes < align" (you can distribute
> >division over addition because offset / align has no reminder, and
> >simplify to "0 == bytes / align").
> >
> >Putting it together, it becomes "bytes > 0 && bytes < align", or even
> >"bytes < align".
> 
> Oh, thanks, and the if block also too complicated. If I am right it should
> collapse to:
> 
>         if (bytes < align) {
>             qemu_iovec_add(&local_qiov, head_buf + bytes,
>                            align - bytes);
>             bytes = align;
>         }
> 
> Right?

Looks good to me.

Another mostly unrelated thing I just noticed while looking at this
code: Should we assert(is_power_of_2(align)) somewhere?

Kevin