Re: [Qemu-devel] [RFC PATCH] Fake machine for scalability testing

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/20/2011 11:12 AM, Markus Armbruster wrote:
> Anthony Liguori<anthony@codemonkey.ws>  writes:
>
>    
>> On 01/18/2011 02:16 PM, Markus Armbruster wrote:
>>      
>>> The problem: you want to do serious scalability testing (1000s of VMs)
>>> of your management stack.  If each guest eats up a few 100MiB and
>>> competes for CPU, that requires a serious host machine.  Which you don't
>>> have.  You also don't want to modify the management stack at all, if you
>>> can help it.
>>>
>>> The solution: a perfectly normal-looking QEMU that uses minimal
>>> resources.  Ability to execute any guest code is strictly optional ;)
>>>
>>> New option -fake-machine creates a fake machine incapable of running
>>> guest code.  Completely compiled out by default, enable with configure
>>> --enable-fake-machine.
>>>
>>> With -fake-machine, CPU use is negligible, and memory use is rather
>>> modest.
>>>
>>> Non-fake VM running F-14 live, right after boot:
>>> UID        PID  PPID  C    SZ   RSS PSR STIME TTY          TIME CMD
>>> armbru   15707  2558 53 191837 414388 1 21:05 pts/3    00:00:29 [...]
>>>
>>> Same VM -fake-machine, after similar time elapsed:
>>> UID        PID  PPID  C    SZ   RSS PSR STIME TTY          TIME CMD
>>> armbru   15742  2558  0 85129  9412   0 21:07 pts/3    00:00:00 [...]
>>>
>>> We're using a very similar patch for RHEL scalability testing.
>>>
>>>        
>> Interesting, but:
>>
>>   9432 anthony   20   0  153m  14m 5384 S    0  0.2   0:00.22
>> qemu-system-x86
>>
>> That's qemu-system-x86 -m 4
>>      
> Sure you ran qemu-system-x86 -fake-machine?
>    

No, I didn't try it.  My point was that -m 4 is already pretty small.

>> In terms of memory overhead, the largest source is not really going to
>> be addressed by -fake-machine (l1_phys_map and phys_ram_dirty).
>>      
> git-grep phys_ram_dirty finds nothing.
>    

Yeah, it's now ram_list[i].phys_dirty.

l1_phys_map is (sizeof(void *) + sizeof(PhysPageDesc)) * mem_size_in_pages

phys_dirty is mem_size_in_pages bytes.

>> I don't really understand the point of not creating a VCPU with KVM.
>> Is there some type of overhead in doing that?
>>      
> I briefly looked at both main loops, TCG's was the first one I happened
> to crack, and I didn't feel like doing both then.  If the general
> approach is okay, I'll gladly investigate how to do it with KVM.
>    

I guess what I don't understand is why do you need to not run guest 
code?  Specifically, if you remove the following, is it any less useful?

diff --git a/cpu-exec.c b/cpu-exec.c
index 8c9fb8b..cd1259a 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -230,7 +230,7 @@ int cpu_exec(CPUState *env1)
      uint8_t *tc_ptr;
      unsigned long next_tb;

-    if (cpu_halted(env1) == EXCP_HALTED)
+    if (fake_machine || cpu_halted(env1) == EXCP_HALTED)

          return EXCP_HALTED;


Regards,

Anthony Liguori