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Re: [Qemu-devel] emulated ARM performance vs real processor ?

From: Julien Heyman
Subject: Re: [Qemu-devel] emulated ARM performance vs real processor ?
Date: Fri, 2 Sep 2011 18:04:37 +0200

Thanks Dave.
I use system emulation, and my main concern is "just" to know that the actual board will run faster than the emulation. So based on your example, and even though my target board (mini2440) is nowhere as fast as a Panda board, this should be the case by a comfortable margin. Now, as I am focusing on boot time, the time to read from flash (i.e. much faster in the emulated context than on the real flash) will counter-balance this a lot. Hopefully these two factors will even out and what I measure now will not be dramatically different than what I will get on the real board, but...we'll see.


On Fri, Sep 2, 2011 at 4:31 PM, David Gilbert <address@hidden> wrote:
On 1 September 2011 08:32, Julien Heyman <address@hidden> wrote:
> Hi,
> I was wondering if anyone had some data regarding the relative performance
> of any given ARM board emulated in QEMU versus the real thing. Yes, I do
> know this depends a lot on the host PC running qemu, but some
> ballpark/example figures would help. Say, I emulate a 400 Mhz ARM9 processor
> on a Core2Duo laptop @ 2 Ghz, what kind of performance/timing ratio should I
> expect, one way or the other ? For example, for boot time.
> I have no idea whether the overhead of emulation is over-compensated by the
> huge processing power of the host compared to the real HW target, and by
> which factor.

Comparing performance is always a bit tricky, and I've not really got
a solid set of benchmarks
ready to run to try it but to give some numbers:

1) Boot times
  Comparing the Linaro 11.08 ubuntu desktop images, time to boot to desktop

  Real Panda board (dual core A9 at 1GHz, 1GB RAM, running off SD
card) - 2minutes to desktop
  QEMU vexpress (2xA9 core, 1GB RAM, emulated sd card, running on a
Core2 Duo T9400 2.53GHz laptop) - 3minutes to desktop

  (The times are scarily close to exact minutes - timeout somewhere?)
  Now, QEMU system mode only ever uses one host core when emulating
multiple cores, so there is a factor 2 disadvantage there, but
on the plus side the memory bandwidth of the host and the disk speed
is probably much higher than the Panda.

2) Simple md5sum benchmark
  As a really simple benchmark the test:

   time (dd if=/dev/zero bs=1024k count=1000 | md5sum)

   Panda board 14.5s real, 10.7 user, 3.8s system
   Emulated Overo board (single A8 processor on same laptop as above)
- 41s real, 24.7s user, 16.4s system
   User mode emulated - 14.2s real, 14s user, 0.5s system
   Native on x86 host - 3.2s real, 2.5s user, 1.2s system

So, that's two sets of pretty bogus dummy simple benchmarks!

I suppose one observation is that the boot time isn't that bad
compared to the real (different) hardware, the user mode emulation
was comparable to the Panda, but the system emulation on a simple test
seems a lot slower.

These things will vary wildly depending what your benchmark is; but as
a summary I'd say that the ARM system mode emulation is
fast enough to use interactively but CPU wise is noticeably slower
than user mode emulation.


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