On 17 February 2014 03:32, Gleb Smirnoff <gleb...@freebsd.org> wrote: > On Mon, Feb 17, 2014 at 11:24:21AM +0000, David Chisnall wrote: > D> > Now for the new sendfile. The core idea is that sendfile() > D> > schedules the I/O, but doesn't wait for it to complete. It > D> > returns immediately to the process, and I/O completion is > D> > processed in kernel context. Unlike aio(4), no additional > D> > threads in kernel are created. The new sendfile is a drop-in > D> > replacement for the old one. Applications (like nginx) doesn't > D> > need recompile, neither configuration change. The SF_NODISKIO is > D> > ignored. > D> > D> Doesn't this introduce a race? If I do a sendfile now, then I am at > liberty to modify the underlying file as soon as it returns. With this > version, I not only am not free to modify the file, I have no notification > that it is finished so I can't ever safely use this call on a file that I > might eventually modify. > D> > D> Wouldn't it be better to provide an aio_sendfile() that would deliver > completion notifications via the normal aio mechanism? > > This race actually exists with the classical sendfile and existed > ever since. While pages are in socket buffer, and not yet sent > out wire, any modification to them would affect data sent. > > To know when you can modify data you need to use SF_SYNC flag. > The flag should work with new sendfile as well, effectively > making it blocking on socket. I haven't tested this, but > should work. > > Adrian have also committed the kqueue notification of SF_SYNC > event. This should work in combination with new sendfile, but > that hasn't been tested yet. This is going to be tested soon, > since we need this functionality.
Yup, that's exactly why I committed the sendfile kqueue completion notification stuff. It's main caveat is that it requires that the ethernet driver immediately free the mbuf upon TX completion - which the chelsio driver doesn't yet do. That has to be fixed for the notification completion stuff to behave well under lower traffic loads. -a _______________________________________________ email@example.com mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"