8.1-STABLE: zfs and sendfile: problem still exists

[Alexander Zagrebin]

> > I've tried the nginx with
> > disabled sendfile (the nginx.conf contains "sendfile off;"):
> > 
> > $ dd if=/dev/random of=test bs=1m count=100
> > 100+0 records in
> > 100+0 records out
> > 104857600 bytes transferred in 5.892504 secs (17795083 bytes/sec)
> > $ fetch -o /dev/null http://localhost/test
> > /dev/null                                     100% of  100 
> MB   41 MBps
> > $ fetch -o /dev/null http://localhost/test
> > /dev/null                                     100% of  100 
> MB   44 MBps
> > $ fetch -o /dev/null http://localhost/test
> > /dev/null                                     100% of  100 
> MB   44 MBps
> > 
> 
> I am really surprised with such a bad performance of sendfile.
> Will you be able to profile the issue further?

Yes.

> I will also try to think of some measurements.

A transfer rate is too low for the _first_ attempt only.
Further attempts demonstrates a reasonable transfer rate.
For example, nginx with "sendfile on;":

$ dd if=/dev/random of=test bs=1m count=100
100+0 records in
100+0 records out
104857600 bytes transferred in 5.855305 secs (17908136 bytes/sec)
$ fetch -o /dev/null http://localhost/test
/dev/null                                       3% of  100 MB  118 kBps
13m50s^C
fetch: transfer interrupted
$ fetch -o /dev/null http://localhost/test
/dev/null                                     100% of  100 MB   39 MBps

If there was no access to the file during some time, then everything
repeats:
The first attempt - transfer rate is too low
A further attempts - no problems

Can you reproduce the problem on your system?

-- 
Alexander Zagrebin