Runtime discrepancy clock_gettime + funcition instrumentation

[Christian Weiss]

Dear Florian,

I checked on a newer Ubuntu system with libc version 2.33. There, I can confirm that vdso is used and that the static binary is in fact faster. Thanks for your helping out. 

-----Original Message-----
From: Florian Weimer [mailto:fweimer@redhat.com] 
Sent: 29 July 2021 10:21
To: Christian Weiss <Christian.Weiss@EMEA.NEC.COM>
Cc: libc-help@sourceware.org; Felix Uhl <Felix.Uhl@EMEA.NEC.COM>
Subject: Re: Runtime discrepancy clock_gettime + funcition instrumentation

* Christian Weiss:

> Dear Florian,
>
> Thank you for the quick reply. As far as I understand, I can check if vdso is used by using strace on the binary. Doing so I get:
>
> ```[cweiss@amd022 overhead]$ strace ./test_time_static_gcc.x 1 2>&1 | 
> grep -i clock_gettime clock_gettime(CLOCK_MONOTONIC, {tv_sec=89045, 
> tv_nsec=95268751}) = 0 clock_gettime(CLOCK_MONOTONIC, {tv_sec=89145, 
> tv_nsec=95813405}) = 0 clock_gettime(CLOCK_MONOTONIC, {tv_sec=89145, 
> tv_nsec=95863144}) = 0 clock_gettime(CLOCK_MONOTONIC, {tv_sec=89145, 
> tv_nsec=95933573}) = 0
> [cweiss@amd022 overhead]$ strace ./test_time_dyn_gcc.x 1 2>&1 | grep 
> -i clock_gettime```
>
> Am I right that this implies that vdso is not used in the static 
> binary, but in the dynamic one? Is there a way to activate it in the 
> static case or do I just have to live with that?

Later glibc versions support vDSO in static binaries.  glibc 2.17 as used by Red Hat Enterprise Linux 7 apparently does not.  (Your observations match my recollections, but I haven't double-checked them.)

Thanks,
Florian



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