Re: [Qemu-devel] [Qemu-block] [RFC PATCH V4] qemu-img: make convert async

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Fri, Feb 17, 2017 at 05:00:24PM +0100, Peter Lieven wrote:
> this is something I have been thinking about for almost 2 years now.
> we heavily have the following two use cases when using qemu-img convert.
> 
> a) reading from NFS and writing to iSCSI for deploying templates
> b) reading from iSCSI and writing to NFS for backups
> 
> In both processes we use libiscsi and libnfs so we have no kernel pagecache.
> As qemu-img convert is implemented with sync operations that means we
> read one buffer and then write it. No parallelism and each sync request
> takes as long as it takes until it is completed.
> 
> This is version 4 of the approach using coroutine worker "threads".
> 
> So far I have the following runtimes when reading an uncompressed QCOW2 from
> NFS and writing it to iSCSI (raw):
> 
> qemu-img (master)
>  nfs -> iscsi 22.8 secs
>  nfs -> ram   11.7 secs
>  ram -> iscsi 12.3 secs
> 
> qemu-img-async (8 coroutines, in-order write disabled)
>  nfs -> iscsi 11.0 secs
>  nfs -> ram   10.4 secs
>  ram -> iscsi  9.0 secs
> 
> The following are the runtimes found with different settings between V3 and V4.
> This is always the best runtime out of 10 runs when converting from nfs to iscsi.
> Please note that in V4 in-order write scenarios show a very high jitter. I think
> this is because the get_block_status on the NFS share is delayed by concurrent read
> requests.
> 
>                        in-order        out-of-order
> V3  - 16 coroutines    12.4 seconds    11.1 seconds
>     -  8 coroutines    12.2 seconds    11.3 seconds
>     -  4 coroutines    12.5 seconds    11.1 seconds
>     -  2 coroutines    14.8 seconds    14.9 seconds
> 
> V4  - 32 coroutines    15.9 seconds    11.5 seconds
>     - 16 coroutines    12.5 seconds    11.0 seconds
>     -  8 coroutines    12.9 seconds    11.0 seconds
>     -  4 coroutines    14.1 seconds    11.5 seconds
>     -  2 coroutines    16.9 seconds    13.2 seconds

Does this patch work with compressed images?  Especially the
out-of-order write mode may be problematic with a compressed qcow2 image.

How should a user decide between in-order and out-of-order?

> @@ -1651,12 +1680,117 @@ static int convert_write(ImgConvertState *s, int64_t sector_num, int nb_sectors,
>      return 0;
>  }
>  
> -static int convert_do_copy(ImgConvertState *s)
> +static void convert_co_do_copy(void *opaque)

Missing coroutine_fn here and for convert_co_read()/convert_co_write().
Functions that must be called from coroutine context (because they
yield, use coroutine mutexes, etc) need to be marked as such.

> +        if (s->wr_in_order) {
> +            /* reenter the coroutine that might have waited
> +             * for this write to complete */
> +            s->wr_offs = sector_num + n;
> +            for (i = 0; i < s->num_coroutines; i++) {
> +                if (s->co[i] && s->wait_sector_num[i] == s->wr_offs) {
> +                    qemu_coroutine_enter(s->co[i]);
> +                    break;

This qemu_coroutine_enter() call relies on the yield pattern between
sibling coroutines having no recursive qemu_coroutine_enter() calls.
QEMU aborts if there is a code path where coroutine A enters B and then
B enters A again before yielding.

Paolo's new aio_co_wake() API solves this issue by deferring the
qemu_coroutine_enter() to the event loop.  It's similar to CoQueue
wakeup.  aio_co_wake() is part of my latest block pull request (should
be merged into qemu.git soon).

I *think* this patch has no A -> B -> A situation thanks to yields in
the code path, but it would be nicer to use aio_co_wake() where this can
never happen.

> +        case 'm':
> +            num_coroutines = atoi(optarg);
> +            if (num_coroutines > MAX_COROUTINES) {
> +                error_report("Maximum allowed number of coroutines is %d",
> +                             MAX_COROUTINES);
> +                ret = -1;
> +                goto fail_getopt;
> +            }

Missing input validation for the < 1 case.

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