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Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutin
From: |
Anthony Liguori |
Subject: |
Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O |
Date: |
Wed, 26 Jan 2011 10:28:15 -0600 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.15) Gecko/20101027 Lightning/1.0b1 Thunderbird/3.0.10 |
On 01/26/2011 10:13 AM, Avi Kivity wrote:
Serializing against a global mutex has the advantage that it can be
treated as a global lock that is decomposed into fine-grained locks.
For example, we can start the code conversion from an explict async
model to a threaded sync model by converting the mutex into a
shared/exclusive lock. Operations like read and write take the lock
for shared access (and take a fine-grained mutex on the metadata cache
entry), while operation like creating a snapshot take the lock for
exclusive access. That doesn't work with freeze/thaw.
The trouble with this is that you increase the amount of re-entrance
whereas freeze/thaw doesn't.
The code from the beginning of the request to where the mutex is
acquired will be executed for every single request even while requests
are blocked at the mutex acquisition.
With freeze/thaw, you freeze the queue and prevent any request from
starting until you thaw. You only thaw and return control to allow
another request to execute when you begin executing an asynchronous I/O
callback.
I think my previous example was wrong, you really want to do:
qcow2_aio_writev() {
coroutine {
freeze();
sync_io(); // existing qcow2 code
thaw();
// existing non I/O code
bdrv_aio_writev(callback); // no explicit freeze/thaw needed
}
}
This is equivalent to our existing code because no new re-entrance is
introduced. The only re-entrancy points are in the
bdrv_aio_{readv,writev} calls.
Regards,
Anthony Liguori
- Re: [Qemu-devel] [RFC][PATCH 05/12] coroutine: Add coroutines, (continued)
- [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Stefan Hajnoczi, 2011/01/22
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Anthony Liguori, 2011/01/23
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/26
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Kevin Wolf, 2011/01/26
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Anthony Liguori, 2011/01/26
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/26
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O,
Anthony Liguori <=
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/26
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Anthony Liguori, 2011/01/26
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/27
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Kevin Wolf, 2011/01/27
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/27
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Kevin Wolf, 2011/01/27
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/27
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Kevin Wolf, 2011/01/27
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/27
- Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O, Avi Kivity, 2011/01/26