From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=56782 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Pi7bf-0006Pg-Kj for qemu-devel@nongnu.org; Wed, 26 Jan 2011 10:48:28 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Pi7be-0005TP-4k for qemu-devel@nongnu.org; Wed, 26 Jan 2011 10:48:27 -0500 Received: from mx1.redhat.com ([209.132.183.28]:19406) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Pi7bd-0005TG-UD for qemu-devel@nongnu.org; Wed, 26 Jan 2011 10:48:26 -0500 Message-ID: <4D4042A8.2040903@redhat.com> Date: Wed, 26 Jan 2011 16:50:00 +0100 From: Kevin Wolf MIME-Version: 1.0 Subject: Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O References: <1295688567-25496-1-git-send-email-stefanha@linux.vnet.ibm.com> <1295688567-25496-12-git-send-email-stefanha@linux.vnet.ibm.com> <4D40406B.2070302@redhat.com> In-Reply-To: <4D40406B.2070302@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: Anthony Liguori , Stefan Hajnoczi , qemu-devel@nongnu.org Am 26.01.2011 16:40, schrieb Avi Kivity: > On 01/22/2011 11:29 AM, Stefan Hajnoczi wrote: >> Converting qcow2 to use coroutines is fairly simple since most of qcow2 >> is synchronous. The synchronous I/O functions likes bdrv_pread() now >> transparently work when called from a coroutine, so all the synchronous >> code just works. >> >> The explicitly asynchronous code is adjusted to repeatedly call >> qcow2_aio_read_cb() or qcow2_aio_write_cb() until the request completes. >> At that point the coroutine will return from its entry function and its >> resources are freed. >> >> The bdrv_aio_readv() and bdrv_aio_writev() user callback is now invoked >> from a BH. This is necessary since the user callback code does not >> expect to be executed from a coroutine. >> >> This conversion is not completely correct because the safety the >> synchronous code does not carry over to the coroutine version. >> Previously, a synchronous code path could assume that it will never be >> interleaved with another request executing. This is no longer true >> because bdrv_pread() and bdrv_pwrite() cause the coroutine to yield and >> other requests can be processed during that time. >> >> The solution is to carefully introduce checks so that pending requests >> do not step on each other's toes. That is left for a future patch... > > The way I thought of doing this is: > > qcow_aio_write(...) > { > execute_in_coroutine { > co_mutex_lock(&bs->mutex); > do_qcow_aio_write(...); // original qcow code > co_mutex_release(&bs->mutex); > } > } > > (similar changes for the the other callbacks) > > if the code happens to be asynchronous (no metadata changes), we'll take > the mutex and release it immediately after submitting the I/O, so no > extra serialization happens. If the code does issue a synchronous > metadata write, we'll lock out all other operations on the same block > device, but still allow the vcpu to execute, since all the locking > happens in a coroutine. > > Essentially, a mutex becomes the dependency tracking mechnism. A global > mutex means all synchronous operations are dependent. Later, we can > convert the metadata cache entry dependency lists to local mutexes > inside the cache entry structures. I thought a bit about it since you mentioned it in the call yesterday and I think this approach makes sense. Even immediately after the conversion we should be in a better state than with Stefan's approach because I/O without metadata disk access won't be serialized. In the other thread you mentioned that you have written some code independently. Do you have it in some public git repository? Kevin