Re: [Qemu-devel] [PATCH 2/9] S/390 CPU emulation

[Ulrich Hecht]

On Monday 02 November 2009, Laurent Desnogues wrote:
> That indeed looks strange:  fixing the TB chaining on ARM
> made nbench i386 three times faster.  Note the gain was
> less for FP parts of the benchmark due to the use of
> helpers.
>
> out of curiosity could you post your tb_set_jmp_target1
> function?

I'm on an AMD64 host, so it's the same code as in mainline.

> The only thing I can think of at the moment that 
> could make the code slower is that the program you ran
> was not reusing blocks and/or cache flushing in
> tb_set_jmp_target1 is overkill.

There is no cache flushing in the AMD64 tb_set_jmp_target1() function, 
and the polarssl test suite is by nature rather repetitive.

I did some experiments, and it seems disabling the TB chaining (by 
emptying tb_set_jmp_target()) does not have any impact on performance at 
all on AMD64. I tested it with several CPU-intensive programs (md5sum 
and the like) with AMD64 on AMD64 userspace emulation (qemu-x86_64), and 
the difference in performance with TB chaining and without is hardly 
measurable. The chaining is performed as advertised if enabled, I 
checked that, but it does not seem to help performance.

How is this possible? Could this be related to cache size? I suspect the 
Phenom 9500 of mine is better equipped in that area than the average ARM 
controller.

And does the TB chaining actually work on AMD64 at all? I checked by 
adding some debug output, and it seems to patch the jumps correctly, but 
maybe somebody can verify that.

CU
Uli

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