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

[Paolo Bonzini]

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(_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

[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(_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

[Peter Lieven]

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(_qiov, head_buf + bytes,
   align - bytes);
bytes = align;
}

Right?

Thanks,
Peter

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

[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".

Thanks,

Paolo

> +size_t tail_offs;
> +tail_offs = (offset + bytes) & (align - 1);
> +qemu_iovec_add(_qiov, head_buf + tail_offs,
> +   align - tail_offs);
> +bytes += align - tail_offs;
> +}

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

[Peter Lieven]

in a read-modify-write cycle a small request might cause
head and tail to fall into the same alignment. Currently
QEMU reads the same block twice in this case which is
not necessary.

Signed-off-by: Peter Lieven 
---
 block/io.c | 12 
 1 file changed, 12 insertions(+)

diff --git a/block/io.c b/block/io.c
index 60a6bd8..fa40121 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1430,6 +1430,18 @@ int coroutine_fn bdrv_co_pwritev(BlockDriverState *bs,
 
 bytes += offset & (align - 1);
 offset = offset & ~(align - 1);
+
+/* 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) &&
+offset / align == (offset + bytes) / align) {
+size_t tail_offs;
+tail_offs = (offset + bytes) & (align - 1);
+qemu_iovec_add(_qiov, head_buf + tail_offs,
+   align - tail_offs);
+bytes += align - tail_offs;
+}
 }
 
 if ((offset + bytes) & (align - 1)) {
-- 
1.9.1