Re: system call time increase when turning on CONFIG_PARAVIRT

[Jeremy Fitzhardinge <jeremy@goop.org>]

Tim Chen wrote:
> With CONFIG_PARAVIRT turned on, I've found that time invoking
> system_call jumped up quite a lot.  Using TCP streaming test as a
> workload and running on 32-bit 2.6.20 kernel, system_call goes up from
> 0.00025% all the way to 1.6% in the oprofile data.  There is a drop of
> about 4% in overall throughput for this particular workload. 
>
> With lmbench's null system call test, the call time goes up from 0.10
> usec to 0.225 usec.
>
> I'm testing on dual socket Intel core 2 processor running at 2.67 GHz
> with 4 GB RAM.
[ I assume you're talking about running on native hardware. ]

In the current paravirt changes in the kernel, many of the
paravirtualized operations are implemented as (expensive) indirect calls
via paravirt_ops.  Among the changes in the paravirt patches I posted
yesterday is an enhanced patching mechanism which inlines a lot of the
common operations, and converts the rest into direct calls.

I haven't done any detailed measurements on what effect this will have,
but it does bring the actual executed instruction stream much closer to
the !CONFIG_PARAVIRT case, and so I would hope it would recover most or
all of the performance loss you've noticed.

    J