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

[david.goulet@polymtl.ca (David Goulet)]

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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

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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