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Re: [Qemu-devel] [PATCH] iscsi: partly avoid iovec linearization in iscs
From: |
ronnie sahlberg |
Subject: |
Re: [Qemu-devel] [PATCH] iscsi: partly avoid iovec linearization in iscsi_aio_writev |
Date: |
Mon, 19 Nov 2012 07:56:17 -0800 |
On Mon, Nov 19, 2012 at 7:18 AM, Paolo Bonzini <address@hidden> wrote:
> Il 19/11/2012 15:58, Peter Lieven ha scritto:
>>
>> - /* XXX we should pass the iovec to write16 to avoid the extra copy */
>> - /* this will allow us to get rid of 'buf' completely */
>> size = nb_sectors * BDRV_SECTOR_SIZE;
>> - acb->buf = g_malloc(size);
>> - qemu_iovec_to_buf(acb->qiov, 0, acb->buf, size);
>> + data.size = size;
>> +
>> + /* if the iovec only contains one buffer we can pass it directly */
>> + if (acb->qiov->niov == 1) {
>> + acb->buf = NULL;
>> + data.data = acb->qiov->iov[0].iov_base;
>> + } else {
>> + acb->buf = g_malloc(size);
>> + qemu_iovec_to_buf(acb->qiov, 0, acb->buf, size);
>> + data.data = acb->buf;
>> + }
>
> Looks good, but how hard is it to get rid of the bounce buffer
> completely, as mentioned in the comment? Ronnie?
>
I am working on that, but I will need the thanksgiving weekend to
finish it and test it.
It also means breaking the API, since it would introduce new
functionality so it will take a little while after finishing the
libiscsi part before we could/should introduce it in qemu.
The vast majority of writes seems to be a vector with only a single
element, so Peters change is a good optimization for the common case,
until I get the proper fix finished and tested and pushed in a new
release of libiscsi.
I.e. I think Peters change is good for now. It solves the problem
with "extra memcpy" for the majority of writes until we can get the
full solution ready.
> Paolo