| Message ID | 1464097151-19479-1-git-send-email-pl@kamp.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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; > + }
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
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
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
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(&local_qiov, head_buf + tail_offs, + align - tail_offs); + bytes += align - tail_offs; + } } if ((offset + bytes) & (align - 1)) {
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 <pl@kamp.de> --- block/io.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)