Re: [Qemu-devel] Debian 7.8.0 SPARC64 on qemu - anything i can do to speedup the emulation?

[Artyom Tarasenko]

Hi Richard,

On Sun, Aug 23, 2015 at 2:41 AM, Richard Henderson <address@hidden> wrote:
> On Aug 22, 2015 9:45 AM, Artyom Tarasenko <address@hidden> wrote:
>> For my test case tcg-indirect brings more performance gain than for Dennis:
>>
>> git master: 18m31s
>> tcg-indirect: 16m50s
>> #undef  USE_TCG_OPTIMIZATIONS: 14m18s
>
> Thanks.  That's useful.
>
>>
>>
>> JIT statistic, before starting the test:
>> (qemu) info jit
>> Translation buffer state:
>> gen code size       31851136/314448896
>> TB count            128224/2457592
>> TB avg target size  18 max=704 bytes
>> TB avg host size    248 bytes (expansion ratio: 13.4)
>> cross page TB count 0 (0%)
>> direct jump count   83840 (65%) (2 jumps=64730 50%)
>>
>> Statistics:
>> TB flush count      5
>> TB invalidate count 317160
>> TLB flush count     1180769
>> [TCG profiler not compiled]
>>
>> After
>> (qemu) info jit
>> Translation buffer state:
>> gen code size       282903344/314448896
>> TB count            1139744/2457592
>> TB avg target size  17 max=704 bytes
>> TB avg host size    248 bytes (expansion ratio: 14.0)
>> cross page TB count 0 (0%)
>> direct jump count   739828 (64%) (2 jumps=569074 49%)
>>
>> Statistics:
>> TB flush count      5
>> TB invalidate count 324362
>> TLB flush count     2050744
>>
>> So, TB invalidate count gained only ~ 5000.
>> Yet tcg_optimize is ~7% in the perf top, and tcg_liveness_analysis
>> ~3%. Why do we translate so much?
>
> I don't know.  It must be something SPARC specific, as I don't see so much 
> for alpha.

After some debugging I think it's caused by memory faults. On every
MMU miss / access fault
TB is re-translated multiple times till the faulting instruction is found.

This happens gen_intermediate_code_internal when it's called with spc==true.

AFAICT we produce data/access faults only on load/store instructions, i.e.
if GET_FIELD(insn, 0, 1)  == 3. Can this knowledge be used to reduce
the number of re-translations?

Artyom

-- 
Regards,
Artyom Tarasenko

SPARC and PPC PReP under qemu blog: http://tyom.blogspot.com/search/label/qemu