Re: grep-3.7.98-c9ac on Linux/s390x

[Bruno Haible]

Hi Jim,

> > On Linux/s390x (emulated with qemu-system-s390x) all tests pass, except one:
> >
> >
> > FAIL: long-pattern-perf
> > =======================
> >
> > ...
> > + base_ms=1250
> > ++ user_time_ 1 grep -f re-10x in
> > ...
> > + b10x_ms=81800
> > + returns_ 1 expr 1250 '<' 81800 / 20
> > 1
> > + fail=1
> > + Exit 1
> > + set +e
> > + exit 1
> > + exit 1
> > + remove_tmp_
> > + __st=1
> > + cleanup_
> > + :
> > + test '' = yes
> > + cd /home/bruno/grep-3.7.98-c9ac/build-64/tests
> > + chmod -R u+rwx 
> > /home/bruno/grep-3.7.98-c9ac/build-64/tests/gt-long-pattern-perf.PZ1I
> > + rm -rf 
> > /home/bruno/grep-3.7.98-c9ac/build-64/tests/gt-long-pattern-perf.PZ1I
> > + exit 1
> > FAIL long-pattern-perf (exit status: 1)
> >
> >
> > Apparently the timings are not so reliable in a qemu emulated system.
> 
> Thanks for reporting that.
> Is that reproducible?

Yes. In a second run, the test fails again, this time with

+ base_ms=1120
++ user_time_ 1 grep -f re-10x in
...
+ b10x_ms=38550
+ returns_ 1 expr 1120 '<' 38550 / 20

> That test allows some leeway, letting a 10x increase in regex size
> evoke a 20x runtime increase.
> Accommodating this fringe environment (at least with that one sample
> timing) would require raising that well past 60x, which would get too
> close to the old 100x perf hit that's supposed to be avoided, now.

I agree that the thresholds that you have chosen are reasonable.

Apparently, qemu is special due to its built-in just-in-time compiler
technology.

Is there a way to detect this special situation? The tips from [1],
or these conditions:

LC_ALL=C mount | grep '^/dev/vda.* on / ' >/dev/null
LC_ALL=C lscpu | grep '^Virtualization type:.*full' >/dev/null

detect system-level virtualization, but cannot distinguish an
emulated CPU from a paravirtualized CPU. Even when considering

cat /proc/cpuinfo | grep -i bogomips

So, I'm not able to propose a patch to detect this special situation.

Bruno

[1] 
https://unix.stackexchange.com/questions/89714/easy-way-to-determine-the-virtualization-technology-of-a-linux-machine