Re: [PATCH v7 2/5] QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX

[Leonardo Bras Soares Passos]

On Thu, Jan 13, 2022 at 7:34 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Thu, Jan 13, 2022 at 10:06:14AM +0000, Daniel P. Berrangé wrote:
> > On Thu, Jan 13, 2022 at 02:48:15PM +0800, Peter Xu wrote:
> > > On Thu, Jan 06, 2022 at 07:13:39PM -0300, Leonardo Bras wrote:
> > > > @@ -558,15 +575,26 @@ static ssize_t 
> > > > qio_channel_socket_writev(QIOChannel *ioc,
> > > >          memcpy(CMSG_DATA(cmsg), fds, fdsize);
> > > >      }
> > > >
> > > > +    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> > > > +        sflags = MSG_ZEROCOPY;
> > > > +    }
> > > > +
> > > >   retry:
> > > > -    ret = sendmsg(sioc->fd, &msg, 0);
> > > > +    ret = sendmsg(sioc->fd, &msg, sflags);
> > > >      if (ret <= 0) {
> > > > -        if (errno == EAGAIN) {
> > > > +        switch (errno) {
> > > > +        case EAGAIN:
> > > >              return QIO_CHANNEL_ERR_BLOCK;
> > > > -        }
> > > > -        if (errno == EINTR) {
> > > > +        case EINTR:
> > > >              goto retry;
> > > > +        case ENOBUFS:
> > > > +            if (sflags & MSG_ZEROCOPY) {
> > > > +                error_setg_errno(errp, errno,
> > > > +                                 "Process can't lock enough memory for 
> > > > using MSG_ZEROCOPY");
> > > > +                return -1;
> > > > +            }
> > >
> > > I have no idea whether it'll make a real differnece, but - should we 
> > > better add
> > > a "break" here?  If you agree and with that fixed, feel free to add:
> > >
> > > Reviewed-by: Peter Xu <peterx@redhat.com>
> > >
> > > I also wonder whether you hit ENOBUFS in any of the environments.  On 
> > > Fedora
> > > here it's by default unlimited, but just curious when we should keep an 
> > > eye.
> >
> > Fedora doesn't allow unlimited locked memory by default
> >
> > $ grep "locked memory" /proc/self/limits
> > Max locked memory         65536                65536                bytes
> >
> > And  regardless of Fedora defaults, libvirt will set a limit
> > for the guest. It will only be unlimited if requiring certain
> > things like VFIO.
>
> Thanks, I obviously checked up the wrong host..
>
> Leo, do you know how much locked memory will be needed by zero copy?  Will
> there be a limit?  Is it linear to the number of sockets/channels?

It depends on the number of channels, of course, but there are
influencing factors, like network bandwidth & usage, and cpu speed &
usage, network queue size, VM pagesize and so on.

A simple exemple:
If the cpu is free/fast, but there are other applications using the
network, we may enqueue a lot of stuff for sending, and end up needing
a lot of locked memory.

I don't think it's easy to calculate a good reference value for locked
memory here.

>
> It'll be better if we can fail at enabling the feature when we detected that
> the specified locked memory limit may not be suffice.

I agree it's a good idea. But having this reference value calculated
is not much simple, IIUC.

>
> --
> Peter Xu
>