Re: [Qemu-devel] time inside qemu

[Marius Monton <Marius.Monton@uab.cat>]

[-- Attachment #1.1: Type: text/plain, Size: 2693 bytes --]



En/na Paul Brook ha escrit:
> On Monday 16 April 2007 15:41, Marius Monton wrote:
>   
>>> Any benchmark/performance measurements you make inside qemu are
>>> meaningless. qemu performance bears no relation whatsoever to the
>>> performance characteristics of real hardware.
>>>       
>> That's true, and I don't care about it. I'd like to get a method to
>> stop/start time inside qemu in order to simulate execution of large
>> pieces of hw out of qemu (look at qemu-systemc project).
>> If qemu is freeze  meanwhile a systemc simulation is in progress
>> (simulating a HW device of system), time should be freeze also.
>> In this way, execution time of a program inside qemu should appear
>> shorter when using accelerator HW than only SW application. I know these
>> times are not reals, but it should be enough to estimate correctness and
>> execution time on real platforms.
>>     
>
> You're deluding yourself. I simply don't believe you can get any meaningful 
> performance measurements out of qemu. You certainly can't use it to evaluate 
> the correctness of time sensitive algorithms.
>
> qemu execution times can easily be orders of magnitude different from real 
> hardware. i.e. if you have two operations that take the same amount of time 
> to execute on real hardware, one of those operations may take many times 
> longer than the other inside qemu.
>   
It's possible, but I think that roughly speaking, more code implies more
time:

code (a) : for (int i = 0; i< 1000000; i++) c[i] = a[i] * b[i];

code (b) : for (int i = 0; i< 1000; i++) for(int  j = 0; j < b[i]; j++)
c[i] += a[i];

code (c) : for (int i = 0; i< 1000; i++) c[i] = HW_MUL(a[i], b[i]);

I'm sure that code (b) will execute much longer that code (a) inside
qemu (sure that different that in real platform), and I'd like to
compute executing time for code (c) in some way.
So, how can I trap time information/calculation inside qemu?
>  
> If nothing else you're entirely at the mercy of the host OS process scheduler 
> and signal delivery. The emulated CPU may stall for an arbitrary time at any 
> point.
>
>   
Sure, but this can be avoided using multiple simulations and calculating
arithmetic mean for execution time.
> Paul
>
>   

-- 
Màrius Montón i Macián   marius.monton@uab.cat
<mailto:marius.monton@uab.cat>  http://cephis.uab.es
<http://www.mariusmonton.name>
Hardware Engineer
CEPHIS
Centre de Prototips i Solucions Hardware-Software
Dep. Microelectrònica i Sistemes Electrònics
ETSE - Universitat Autònoma de Barcelona (UAB) 	Phone: +34 935 813 534
Fax: +34 935 813 033
QC-2090D. ETSE. Campus UAB.
080193 Bellaterra



[-- Attachment #1.2: Type: text/html, Size: 4260 bytes --]

[-- Attachment #2: marius.monton.vcf --]
[-- Type: text/x-vcard, Size: 350 bytes --]

begin:vcard
fn;quoted-printable:M=C3=A0rius Mont=C3=B3n
n;quoted-printable;quoted-printable:Mont=C3=B3n;M=C3=A0rius
org:CEPHIS-UAB
adr:Campus de la Autonoma;;QC-2090D. ETSE;Bellaterra;Barcelona;08193;Spain
email;internet:marius.monton@uab.cat
title:System Engineer
tel;work:+34935813534
url:http://cephis.uab.cat
version:2.1
end:vcard