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

[Peter Xu]

On Thu, Jan 13, 2022 at 10:42:39AM +0000, Daniel P. Berrangé wrote:
> On Thu, Jan 13, 2022 at 06:34:12PM +0800, Peter Xu 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?
> 
> IIRC we decided it would be limited by the socket send buffer size, rather
> than guest RAM, because writes will block once the send buffer is full.
> 
> This has a default global setting, with per-socket override. On one box I
> have it is 200 Kb. With multifd you'll need  "num-sockets * send buffer".
> 
> > 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.
> 
> Checking this value against available locked memory though will always
> have an error margin because other things in QEMU can use locked memory
> too

We could always still allow false positive in this check, so we can fail if we
have a solid clue to know we'll fail later (e.g. minimum locked_vm needed is
already less than total).  But no strong opinion; we could have this merged and
see whether that's needed in real life.  Thanks,

-- 
Peter Xu