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

[Aurelien Jarno]

On Mon, Nov 09, 2009 at 06:55:23PM +0200, Ulrich Hecht wrote:
> 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.

I have tested it by removing all the block around tb_add_jump in
cpu_exec.c. I have a speed loss of about 2.5x in the boot time of an
x86_64 image.

> 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.

For me it's on a Core 2 Duo T7200, so I doubt it is related to cache
size.

> 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.
> 

Given the gain in speed I have, I guess it works.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net