On Fri, May 10, 2013 at 02:16:34PM +0200, Paolo Bonzini wrote: > Cast debugging can have a substantial cost (20% or more, measured by > Aurelien on qemu-system-ppc64). Instead of adding special-cased "fast > casts" in the hot paths, we can just disable it in releases. At the > same time, add tracing facilities that simplify the analysys of those > problems that cast debugging would reveal. > > At least patches 1-7 are for 1.5. > > Paolo Bonzini (9): > qom: improve documentation of cast functions > qom: allow casting of a NULL class > qom: add a fast path to object_class_dynamic_cast > qom: pass file/line/function to asserting casts > qom: trace asserting casts > qom: allow turning cast debugging off > build: disable QOM cast debugging for official releases > qom: simplify object_class_dynamic_cast, part 1 > qom: simplify object_class_dynamic_cast, part 2 > > configure | 20 ++++++++------ > include/qom/object.h | 40 ++++++++++++++++++++++----- > qom/object.c | 77 > ++++++++++++++++++++++++++++++++++------------------ > trace-events | 3 ++ > 4 files changed, 99 insertions(+), 41 deletions(-) >
I have tested this series with qemu-system-ppc64, on a Core i7 2600 CPU. The process was set to a single core using taskset. Inside the guest (Debian ppc64 from debian-ports), I ran the command three times: lintian g++-4.8_4.8.0-6_ppc64.deb I used lintian as it's a perl code, that trigger the discussion about sparc/ppc comparison. First of all with this patch series, the object_class_dynamic_cast calls went down to below 0.1% when using perf top. Before the patch series was applied, the command took in average on 3 runs 142.4s. With the patch series, it went down to 129.8s, so almost 9% faster. To improve the performance a bit more, and come back to the same kind of code as before, we should move simple accessors from qom/*.c to include/qom/*.h and mark them as inline, so that they can be removed by the compiler. Currently, even if the function is simple it's still a call/ret in the hot path instead of a simple pointer addition. -- Aurelien Jarno GPG: 1024D/F1BCDB73 [email protected] http://www.aurel32.net
