Re: [Qemu-block] [PATCH] block/io: optimize bdrv_co_pwritev for small requests
On 24/05/2016 16:07, Peter Lieven wrote:
>
> 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?
Yes, that should work. But add a comment because it's much more
mysterious than your v1 patch. :)
Even just
/* We have read the tail already if the request is smaller
* than one aligned block.
*/
Thanks,
Paolo
Re: [Qemu-block] [PATCH] block/io: optimize bdrv_co_pwritev for small requests
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
Re: [Qemu-block] [PATCH] block/io: optimize bdrv_co_pwritev for small requests
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?
Thanks,
Peter
Re: [Qemu-block] [PATCH] block/io: optimize bdrv_co_pwritev for small requests
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".
Thanks,
Paolo
> +size_t tail_offs;
> +tail_offs = (offset + bytes) & (align - 1);
> +qemu_iovec_add(&local_qiov, head_buf + tail_offs,
> + align - tail_offs);
> +bytes += align - tail_offs;
> +}
