Re: [uml-user] SMP query

[Suresh <sur...@in...>]

At 06:52 PM 4/26/2005, Vincent Guffens wrote:

>Suresh wrote:
>
>>hi,
>>So in case of uml, if all the interrupts are being handled in the 
>>processor 0, hence
>>softirq and tasklets are going to be handled in processor 0. If this is 
>>the case,
>>then don't you think that smp functionality is lost here. Kindly tell me, 
>>is that the case
>>even in 2.4.20 kernel. In the dairy it was given that it is for 2.5 kernel.
>>Performance may not be an issue to me, so that I can simulate SMP 
>>functionality that
>>host linux offers.
>
>With UML-SMP, you may have multiple processes running simultaneously. For 
>instance, here is a top from an SMP-UML machine:
>
>3:36pm  up 4 min,  4 users,  load average: 0.04, 0.02, 0.00
>  37 processes: 33 sleeping, 4 running, 0 zombie, 0 stopped
>
>     PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND
>     156 root      14   0   376  376   316 R    14.4  1.2   0:01 md5sum
>     155 root      11   0   376  376   316 R    12.6  1.2   0:01 md5sum
>     154 root      13   0   376  376   316 R    12.2  1.2   0:01 md5sum
>     153 root      10   0   920  920   728 R     0.1  3.1   0:00 top
>
>
>But from what I understand, the interrupt paths may run only on processor 
>0 so there is no SMP capability from that point of view. Maybe thinks have 
>changed since I tried. This was with 2.4.24-um1 in tt mode.
>
>However, I don't know if softirq and tasklet may run only on processor 0 
>as interrupts do. Can't these be schedulled on another processor ?

Suppose if interrupts handled only in cpu0. When ever a packet comes to the 
nic, then
isr runs on cpu0. This isr will queue the packet to the cpu0 backlog queue, 
and a softirq is
raised and isr returns. This softirq which is raised on cpu0 will only run 
on cpu0.

In case of tasklets also, if the tasklets are scheduled from interrupts, 
then the will be
will on the processor on which they are scheduled.

How ever if the tasklets are scheduled from the threads, and the threads 
are attached to
another processor, then the scheduled tasklets can run on any other 
processor on which
it is scheduled.
With this most of the kernel functionality seems to run only in cpu0 context.

thanks for the information.

regards
Suresh



>Cheers,
>
>
>--
>                                 Vincent Guffens
>                                 PhD Student UCL/CESAME
>                                 tel:   +32 10 47 80 30
>Value your freedom, or you will lose it, teaches history.
>"Don't bother us with politics," respond those who don't want to learn.
>                 -- Richard M. Stallman