disable sendfile in Varnish, please

[Poul-Henning Kamp]

I've nailed three different operating system kernels as having
sendfile(2) issues today, so I would advice all of you to
disable sendfile to avoid the various problems we've seen.

The easiest way is to specify

	-p sendfile_threshold=-1

to varnishd, or by using the CLI:

	param.set sendfile_threshold -1

Only those of you with high paging rates would be in risk of seing
any performance changes as result of this.

While it is quite easy to set up a test environment to look for
sendfile trouble, we cannot do it as part of varnish runtime so I
think we will ship with sendfile_threshold at -1 for the forseeable
future.

Once we start to see kernels where sendfile works correctly in all
instances, we can consider how to seletively enable those.

Alternatively, if nobody sees a performance loss from disabling it
now, we might as well just forget about it in toto.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.