Re: [PATCH v7 00/25] Unify vDSOs across more architectures

[Andre Przywara]

On Fri, 21 Jun 2019 10:52:27 +0100
Vincenzo Frascino <vincenzo.frascino@xxxxxxx> wrote:

Hi,

> vDSO (virtual dynamic shared object) is a mechanism that the Linux
> kernel provides as an alternative to system calls to reduce where
> possible the costs in terms of cycles.
[ ... ]

Some numbers for the ARM(32) part:

I booted my trusted old Calxeda Midway server (Cortex A-15 cores) and ran
the vdsotest benchmark on it. The results are:
(vdso: times, in nsec/call; n/t: "not tested" (=not implemented))
call				5.2-rc3	5.2-rc3-vdso
clock-gettime-monotonic:        147     142
clock-getres-monotonic:         n/t     34
clock-gettime-monotonic-coarse: 90      96
clock-getres-monotonic-coarse:  n/t     36
clock-gettime-monotonic-raw:    431     142
clock-getres-monotonic-raw:     n/t     35
clock-gettime-tai:              598     150
clock-getres-tai:               n/t     34
clock-gettime-boottime:         592     142
clock-getres-boottime:          n/t     34
clock-gettime-realtime:         149     142
clock-getres-realtime:          n/t     34
clock-gettime-realtime-coarse:  86      96
clock-getres-realtime-coarse:   n/t     36
getcpu:                         n/t     n/t
gettimeofday:                   133     110

So there are some minor improvements, two minor regressions, some
significant improvements (factor 3-4), and some dramatic improvements
(where we actually gained VDSO support).
Overall a pretty impressive outcome for an "Odd fixes" architecture,
especially as it should reduce the future maintenance burden.

Cheers,
Andre.