On Sat, Sep 02, 2017 at 08:17:38PM +0200, Clément Bœsch wrote: > From: Clément Bœsch <cboe...@gopro.com> > > On ARM platforms, accessing the PMU registers requires special user > access permissions. Since there is no other way to get accurate timers, > the current implementation of timers in FFmpeg is using these registers. > Unfortunately, enabling user access to these registers on Linux is not > trivial, and generally involve compiling a random and unreliable github > kernel module, or patching somehow your kernel. > > Such module is very unlikely to reach the upstream anytime soon. Quoting > Robin Murphin from ARM: > > > Say you do give userspace direct access to the PMU; now run two or more > > programs at once that believe they can use the counters for their own > > "minimal-overhead" profiling. Have fun interpreting those results... > > > > And that's not even getting into the implications of scheduling across > > different CPUs, CPUidle, etc. where the PMU state is completely beyond > > userspace's control. In general, the plan to provide userspace with > > something which might happen to just about work in a few corner cases, > > but is meaningless, misleading or downright broken in all others, is to > > never do so. > > As a result, the alternative is to use the Performance Monitoring Linux > API which makes use of these registers internally (assuming the PMU of > your ARM board is supported in the kernel, which is definitely not a > given...). > > While the Linux API is obviously cross platform, it does have a > significant overhead which needs to be taken into account. As a result, > that mode is only weakly enabled on ARM platforms exclusively. > > Note on the non flexibility of the implementation: the timers (native > FFmpeg vs Linux API) are selected at compilation time to prevent the > need of function calls, which would result in a negative impact on the > cycle counters. > --- > configure | 3 ++ > tests/checkasm/checkasm.c | 107 > +++++++++++++++++++++++++++++++++++++--------- > tests/checkasm/checkasm.h | 47 +++++++++++++++++--- > 3 files changed, 132 insertions(+), 25 deletions(-) > > diff --git a/configure b/configure > index 445d953e4f..dc65adfde0 100755 > --- a/configure > +++ b/configure > @@ -448,6 +448,7 @@ Developer options (useful when working on FFmpeg itself): > --libfuzzer=PATH path to libfuzzer > --ignore-tests=TESTS comma-separated list (without "fate-" prefix > in the name) of tests whose result is ignored > + --enable-linux-perf enable Linux Performance Monitor API > > NOTE: Object files are built at the place where configure is launched. > EOF > @@ -1694,6 +1695,7 @@ CONFIG_LIST=" > $SUBSYSTEM_LIST > autodetect > fontconfig > + linux_perf > memory_poisoning > neon_clobber_test > ossfuzz > @@ -5013,6 +5015,7 @@ case $target_os in > linux) > enable dv1394 > enable section_data_rel_ro > + enabled_any arm aarch64 && enable_weak linux_perf > ;; > irix*) > target_os=irix > diff --git a/tests/checkasm/checkasm.c b/tests/checkasm/checkasm.c > index 9173ed19d9..bc5193c6e3 100644 > --- a/tests/checkasm/checkasm.c > +++ b/tests/checkasm/checkasm.c > @@ -20,6 +20,14 @@ > * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. > */ > > +#include "config.h" > +
> +#ifdef CONFIG_LINUX_PERF is this and others intended to be #ifdef instead of #if ? [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB "I am not trying to be anyone's saviour, I'm trying to think about the future and not be sad" - Elon Musk
signature.asc
Description: Digital signature
_______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
