From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46877) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Xye6m-0003uu-7p for qemu-devel@nongnu.org; Wed, 10 Dec 2014 05:03:05 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Xye6h-0003gR-DX for qemu-devel@nongnu.org; Wed, 10 Dec 2014 05:03:00 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43565) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Xye6h-0003gM-6v for qemu-devel@nongnu.org; Wed, 10 Dec 2014 05:02:55 -0500 Date: Wed, 10 Dec 2014 15:32:43 +0530 From: Amit Shah Message-ID: <20141210100242.GE27208@grmbl.mre> References: <1416830152-524-1-git-send-email-arei.gonglei@huawei.com> <1416830152-524-5-git-send-email-arei.gonglei@huawei.com> <20141210031810.GC27208@grmbl.mre> <5487C445.9030506@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5487C445.9030506@huawei.com> Subject: Re: [Qemu-devel] [PATCH RESEND for 2.3 4/6] xbzrle: check 8 bytes at a time after an concurrency scene List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: ChenLiang Cc: weidong.huang@huawei.com, quintela@redhat.com, qemu-devel@nongnu.org, dgilbert@redhat.com, arei.gonglei@huawei.com, pbonzini@redhat.com, peter.huangpeng@huawei.com On (Wed) 10 Dec 2014 [11:55:49], ChenLiang wrote: > On 2014/12/10 11:18, Amit Shah wrote: > > > On (Mon) 24 Nov 2014 [19:55:50], arei.gonglei@huawei.com wrote: > >> From: ChenLiang > >> > >> The logic of old code is correct. But Checking byte by byte will > >> consume time after an concurrency scene. > >> > >> Signed-off-by: ChenLiang > >> Signed-off-by: Gonglei > >> --- > >> xbzrle.c | 28 ++++++++++++++++++---------- > >> 1 file changed, 18 insertions(+), 10 deletions(-) > >> > >> diff --git a/xbzrle.c b/xbzrle.c > >> index d27a140..0477367 100644 > >> --- a/xbzrle.c > >> +++ b/xbzrle.c > >> @@ -50,16 +50,24 @@ int xbzrle_encode_buffer(uint8_t *old_buf, uint8_t *new_buf, int slen, > >> > >> /* word at a time for speed */ > >> if (!res) { > >> - while (i < slen && > >> - (*(long *)(old_buf + i)) == (*(long *)(new_buf + i))) { > >> - i += sizeof(long); > >> - zrun_len += sizeof(long); > >> - } > >> - > >> - /* go over the rest */ > >> - while (i < slen && old_buf[i] == new_buf[i]) { > >> - zrun_len++; > >> - i++; > >> + while (i < slen) { > >> + if ((*(long *)(old_buf + i)) == (*(long *)(new_buf + i))) { > >> + i += sizeof(long); > >> + zrun_len += sizeof(long); > >> + } else { > >> + /* go over the rest */ > >> + for (j = 0; j < sizeof(long); j++) { > >> + if (old_buf[i] == new_buf[i]) { > >> + i++; > >> + zrun_len++; > > > > I don't see how this is different from the code it's replacing. The > > check and increments are all the same. Difficult to see why there'll > > be a speed benefit. Can you please explain? Do you have any > > performance numbers for before/after? > > > > Thanks, > > > > Amit > > > > . > > > > Hi Amit: > > + for (j = 0; j < sizeof(long); j++) { > + if (old_buf[i] == new_buf[i]) { > + i++; > + zrun_len++; > + } else { > + break; > + } > + } > + if (j != sizeof(long)) { > + break; > + } > > The branch of *j != sizeof(long)* may not be hit after an concurrency scene. > so we can continue doing "(*(long *)(old_buf + i)) == (*(long *)(new_buf + i))". > On the another side the old code does "old_buf[i] == new_buf[i]". Frankly, I still don't see it. Earlier: while.. match words while.. match bytes Now: while.. match words if word mismatch match bytes to me, essentially looks the same. I'll propose to drop this patch till we have a proper justification. Amit