From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39962) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XyYO6-0006Tb-Ti for qemu-devel@nongnu.org; Tue, 09 Dec 2014 22:56:35 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XyYO2-0000D8-2m for qemu-devel@nongnu.org; Tue, 09 Dec 2014 22:56:30 -0500 Received: from szxga01-in.huawei.com ([119.145.14.64]:45994) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XyYO1-0000BT-BB for qemu-devel@nongnu.org; Tue, 09 Dec 2014 22:56:26 -0500 Message-ID: <5487C445.9030506@huawei.com> Date: Wed, 10 Dec 2014 11:55:49 +0800 From: ChenLiang MIME-Version: 1.0 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> In-Reply-To: <20141210031810.GC27208@grmbl.mre> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit 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: Amit Shah 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 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]". To be honest, This scene is rare. Best regards ChenLiang