Re: [patch 04/17] Add pagetable accessors to pack and unpack pagetable entries

Ingo Molnar
2007-04-04T09:58:14Z

* Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> > What do the benchmarks say with CONFIG_PARAVIRT on native hardware
> > compared to !CONFIG_PARAVIRT. e.g. does lmbench suffer?
>
> Barely. There's a slight hit for not using patching, and patching is
> almost identical to native performance. The most noticeable
> difference is in the null syscall microbenchmark, but once you get to
> complex things the difference is in the noise.

i'd not call this 'barely' or 'noise'! Look:

> Processor, Processes - times in microseconds - smaller is better
> ------------------------------------------------------------------------------
> Host OS Mhz null null open slct sig sig fork exec sh
> call I/O stat clos TCP inst hndl proc proc proc
> --------- ------------- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
> non-paravirt
> ezr Linux 2.6.21- 1000 0.25 0.52 31.6 34.7 10.3 1.03 5.31 726. 1565 4520
> ezr Linux 2.6.21- 1000 0.25 0.52 31.8 34.7 12.6 1.03 5.41 725. 1564 4585
> ezr Linux 2.6.21- 1000 0.25 0.55 31.7 34.5 11.8 1.02 5.47 720. 1595 4518
>
> paravirt, no patching
> ezr Linux 2.6.21- 1000 0.28 0.55 31.3 34.3 10.0 1.05 5.56 747. 1621 4675
> ezr Linux 2.6.21- 1000 0.28 0.56 31.5 34.3 12.9 1.05 5.66 755. 1629 4684
> ezr Linux 2.6.21- 1000 0.28 0.55 31.8 34.5 12.5 1.05 5.45 747. 1622 4695

the main metric we are interested in is the overhead for people who just
want to run the non-patched native kernel that has CONFIG_PARAVIRT
enabled (99%+ of the users at the moment), so the delta is:

null: +12.0%
null IO: +7.5%
stat: within noise
open/close: within noise
TCP: ~5.0%
signal install: 2.0%
signal handle: 4.7%
fork: 2.7%
exec: 3.6%
shell: 3.6%

this is not 'barely measurable' but 'BLOODY LARGE' overhead. Really. I
mean for something as complex as exec we still are 'a couple of percent'
slower? Linux has literally _dozens_ of important but inactive features
in every critical fastpath and in every critical system call, and if
each took 'just a few percent', we'd be a few _times_ slower as an end
result, for features that are not even used! The answer to any such
overhead is: "get your act together and stop burdening those who dont
want to use that feature" ... This is a basic engineering requirement.

> paravirt, patching
> ezr Linux 2.6.21- 1000 0.25 0.53 31.8 34.4 10.1 1.04 5.44 730. 1583 4600
> ezr Linux 2.6.21- 1000 0.26 0.55 32.1 35.2 13.3 1.03 5.48 748. 1589 4606
> ezr Linux 2.6.21- 1000 0.26 0.54 32.0 34.9 14.1 1.04 5.43 752. 1606 4647

i guess this pretty much makes the case for patching ...

Ingo