In order to help PLE and pvticketlock converge I thought that a small
test code should be developed to test this in a predictable,
deterministic way.
The idea is to have a guest kernel module that spawn a new thread each
time you write to a /sys/.... entry.
Each such a thread spins over a spin lock. The specific spin lock is
also chosen by the /sys/ interface. Let's say we have an array of spin
locks *10 times the amount of vcpus.
All the threads are running a
while (1) {
spin_lock(my_lock);
sum += execute_dummy_cpu_computation(time);
spin_unlock(my_lock);
if (sys_tells_thread_to_die()) break;
}
print_result(sum);
Instead of calling the kernel's spin_lock functions, clone them and make
the ticket lock order deterministic and known (like a linear walk of all
the threads trying to catch that lock).
This way you can easy calculate:
1. the score of a single vcpu running a single thread
2. the score of sum of all thread scores when #thread==#vcpu all
taking the same spin lock. The overall sum should be close as
possible to #1.
3. Like #2 but #threads > #vcpus and other versions of #total vcpus
(belonging to all VMs) > #pcpus.
4. Create #thread == #vcpus but let each thread have it's own spin
lock
5. Like 4 + 2
Hopefully this way will allows you to judge and evaluate the exact
overhead of scheduling VMs and threads since you have the ideal result
in hand and you know what the threads are doing.
My 2 cents, Dor
On 09/21/2012 08:36 PM, Raghavendra K T wrote:
On 09/21/2012 06:48 PM, Chegu Vinod wrote:
On 9/21/2012 4:59 AM, Raghavendra K T wrote:
In some special scenarios like #vcpu <= #pcpu, PLE handler may
prove very costly,
Yes.
because there is no need to iterate over vcpus
and do unsuccessful yield_to burning CPU.
An idea to solve this is:
1) As Avi had proposed we can modify hardware ple_window
dynamically to avoid frequent PL-exit.
Yes. We had to do this to get around some scaling issues for large
(>20way) guests (with no overcommitment)
Do you mean you already have some solution tested for this?
As part of some experimentation we even tried "switching off" PLE too :(
Honestly,
Your this experiment and Andrew Theurer's observations were the
motivation for this patch.
(IMHO, it is difficult to
decide when we have mixed type of VMs).
Agree.
Not sure if the following alternatives have also been looked at :
- Could the behavior associated with the "ple_window" be modified to be
a function of some [new] per-guest attribute (which can be conveyed to
the host as part of the guest launch sequence). The user can choose to
set this [new] attribute for a given guest. This would help avoid the
frequent exits due to PLE (as Avi had mentioned earlier) ?
Ccing Drew also. We had a good discussion on this idea last time.
(sorry that I forgot to include in patch series)
May be a good idea when we know the load in advance..
- Can the PLE feature ( in VT) be "enhanced" to be made a per guest
attribute ?
IMHO, the approach of not taking a frequent exit is better than taking
an exit and returning back from the handler etc.
I entirely agree on this point. (though have not tried above
approaches). Hope to see more expert opinions pouring in.
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