Re: [Qemu-devel] [PATCH v4 1/6] block: use 1 MB bounce buffers for crypto instead of 16KB

[Eric Blake <eblake@redhat.com>]

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On 09/27/2017 07:53 AM, Daniel P. Berrange wrote:
> Using 16KB bounce buffers creates a significant performance
> penalty for I/O to encrypted volumes on storage which high
> I/O latency (rotating rust & network drives), because it
> triggers lots of fairly small I/O operations.
> 
> On tests with rotating rust, and cache=none|directsync,
> write speed increased from 2MiB/s to 32MiB/s, on a par
> with that achieved by the in-kernel luks driver. With
> other cache modes the in-kernel driver is still notably
> faster because it is able to report completion of the
> I/O request before any encryption is done, while the
> in-QEMU driver must encrypt the data before completion.
> 
> Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> ---
>  block/crypto.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>


> @@ -464,12 +467,11 @@ block_crypto_co_writev(BlockDriverState *bs, int64_t sector_num,
>  
>      qemu_iovec_init(&hd_qiov, qiov->niov);
>  
> -    /* Bounce buffer so we have a linear mem region for
> -     * entire sector. XXX optimize so we avoid bounce
> -     * buffer in case that qiov->niov == 1
> +    /* Bounce buffer because we're not permitted to touch
> +     * contents of qiov - it points to guest memory.
>       */
>      cipher_data =
> -        qemu_try_blockalign(bs->file->bs, MIN(BLOCK_CRYPTO_MAX_SECTORS * 512,
> +        qemu_try_blockalign(bs->file->bs, MIN(BLOCK_CRYPTO_MAX_IO_SIZE,
>                                                qiov->size));

We allocate and free a bounce buffer for every write - is there anything
we can do to reduce malloc() calls by reusing a bounce buffer associated
with a coroutine (we have to be careful that parallel coroutines don't
share bounce buffers, of course).  But that's independent of this patch.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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