Re: [PATCH v3 3/3] multifd: Send using asynchronous write on nocomp to send RAM pages.

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Sep 22, 2021 at 07:24:23PM -0300, Leonardo Bras wrote:
> Change multifd nocomp version to use asynchronous write for RAM pages, and
> benefit of MSG_ZEROCOPY when it's available.
> 
> The asynchronous flush happens on cleanup only, before destroying the QIOChannel.
> 
> This will work fine on RAM migration because the RAM pages are not usually freed,
> and there is no problem on changing the pages content between async_send() and
> the actual sending of the buffer, because this change will dirty the page and
> cause it to be re-sent on a next iteration anyway.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  migration/multifd.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 377da78f5b..d247207a0a 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -105,7 +105,7 @@ static int nocomp_send_prepare(MultiFDSendParams *p, uint32_t used,
>   */
>  static int nocomp_send_write(MultiFDSendParams *p, uint32_t used, Error **errp)
>  {
> -    return qio_channel_writev_all(p->c, p->pages->iov, used, errp);
> +    return qio_channel_async_writev_all(p->c, p->pages->iov, used, errp);
>  }

This needs to be made conditional so zeroopy is only used if rquested
by the mgmt app, and it isn't going to work in all cases (eg TLS) so
silently enabling it is not good.

>  
>  /**
> @@ -546,6 +546,7 @@ void multifd_save_cleanup(void)
>          MultiFDSendParams *p = &multifd_send_state->params[i];
>          Error *local_err = NULL;
>  
> +        qio_channel_async_flush(p->c, NULL);
>          socket_send_channel_destroy(p->c);
>          p->c = NULL;
>          qemu_mutex_destroy(&p->mutex);

This isn't reliable beucase qio_channel_async_flush will return early
even if not everything is flushed.

Regards,
Daniel
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