[ltt-dev] [UST] BUG Ubuntu <= Karmic

[Mathieu Desnoyers]

* David Goulet (david.goulet at polymtl.ca) wrote:
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> After some talks in the metro and testing it here at home, we found the bad guy
> in all this mess... it is .... KVM :D

Ah, yes. This is why we need a strategy for a userland trace clock that
takes into account:

- The host kernel
- The guest kernel
- Userspace

Which is transparently available across all three.

Mathieu


> 
> We have no idea yet why but we will look into it.
> 
> Side note to my self : Test on different real host ;)
> 
> Thanks
> David
> 
> On 10-09-16 05:59 PM, David Goulet wrote:
> > Hi everyone,
> > 
> > A very _major_ bug, to say the least, was discover this afternoon in
> > Ubuntu Karmic and below. The clock_gettime and gettimeofday function are
> > syscalls and not VDSO as it suppose to be.
> > 
> > This makes UST go impressively slower because at each tracing event, you
> > got a syscall in the fast path. We got at 2 times factor (per event
> > speed) on Karmic and 5 times on Hardy.
> > 
> > We are currently waiting for feedback from Debian and Ubuntu to explain
> > this changes in the glibc API. (If anybody knows why, please feel free
> > to explain it on this list).
> > 
> > Here is a simple test to see if clock_gettime is in fact a syscall on
> > your distribution (we've only tested Ubuntu distro) :
> > 
> > $ vim test.c
> > 
> > #include <sys/time.h>
> > #include <time.h>
> > 
> > int main(int argc, char **argv) {
> >     struct timespec ts;
> >     clock_gettime(CLOCK_MONOTONIC, &ts);
> >     return 0;
> > }
> > 
> > $ gcc -lrt test.c -o test
> > $ strace ./test
> > 
> > If you see this line in the strace output :
> > clock_gettime(CLOCK_MONOTONIC, ...)
> > 
> > it's a problem.
> > 
> > Thanks to all
> 
> - -- 
> David Goulet
> LTTng project, DORSAL Lab.
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com