From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:49316) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TGQHS-0004LB-Bn for qemu-devel@nongnu.org; Tue, 25 Sep 2012 04:14:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TGQHN-0005Pg-Ti for qemu-devel@nongnu.org; Tue, 25 Sep 2012 04:14:10 -0400 Received: from [222.73.24.84] (port=2318 helo=song.cn.fujitsu.com) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TGQHN-0005Og-I4 for qemu-devel@nongnu.org; Tue, 25 Sep 2012 04:14:05 -0400 Message-ID: <5061691A.9040609@cn.fujitsu.com> Date: Tue, 25 Sep 2012 16:19:38 +0800 From: Wen Congyang MIME-Version: 1.0 References: <1348247243-12446-1-git-send-email-lcapitulino@redhat.com> <1348247243-12446-3-git-send-email-lcapitulino@redhat.com> <505FFD45.2050804@cn.fujitsu.com> <20120924103433.2afcbc0b@doriath.home> In-Reply-To: <20120924103433.2afcbc0b@doriath.home> Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=ISO-8859-1 Subject: Re: [Qemu-devel] [PATCH 2/3] qmp: dump-guest-memory: don't spin if non-blocking fd would block List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Luiz Capitulino Cc: jan.kiszka@siemens.com, aliguori@us.ibm.com, qemu-devel@nongnu.org, d.hatayama@jp.fujitsu.com, armbru@redhat.com At 09/24/2012 09:34 PM, Luiz Capitulino Wrote: > On Mon, 24 Sep 2012 14:27:17 +0800 > Wen Congyang wrote: > >> At 09/22/2012 01:07 AM, Luiz Capitulino Wrote: >>> fd_write_vmcore() will indefinitely spin for a non-blocking >>> file-descriptor that would block. However, if the fd is non-blocking, >>> how does it make sense to spin? >>> >>> Change this behavior to return an error instead. >>> >>> Note that this can only happen with an fd provided by a management >>> application. The fd opened internally by dump-guest-memory is blocking. >>> >>> Signed-off-by: Luiz Capitulino >>> --- >>> dump.c | 13 +++---------- >>> 1 file changed, 3 insertions(+), 10 deletions(-) >>> >>> diff --git a/dump.c b/dump.c >>> index 2bf8d8d..5eea015 100644 >>> --- a/dump.c >>> +++ b/dump.c >>> @@ -100,18 +100,11 @@ static void dump_error(DumpState *s, const char *reason) >>> static int fd_write_vmcore(void *buf, size_t size, void *opaque) >>> { >>> DumpState *s = opaque; >>> - int fd = s->fd; >>> size_t writen_size; >>> >>> - /* The fd may be passed from user, and it can be non-blocked */ >>> - while (size) { >>> - writen_size = qemu_write_full(fd, buf, size); >>> - if (writen_size != size && errno != EAGAIN) { >> >> Hmm, if the fd is a blocking fd, errno can't be EAGAIN. So the >> function doesn't spin. What problems do you meet? > > The problem is with non-blocking fds, where spinning isn't correct, for > two reasons: But, If the fd is non-blocking, errno can't be EAGAIN. So it doesn't spin. Thanks Wen Congyang > > 1. If the fd is non-blocking, that means you don't want to block > and spinning for a long time will have the same effects > > 2. Spinning consumes host resources >