Hi,
On 02/05/2024 15:45, Syed Aftab Rashid wrote:
[snip]
> I just wanted to understand what is the best possible jitter that
I can
> get with Jailhouse. We are working on a project that needs cycle
times
> between 1-2 microseconds. The bare metal implementation of a
similar
> application results in a much lower jitter ~50ns.
So the application does structurally the same like apic-demo?
(i.e., full IRQ arrival and dispatching, "computational task" (i.e.,
calculating jitter), return). 50ns are ~100 TSC cycles – athletic.
I really doubt those latencies are achievable with IRQs. With active
waiting for events (i.e., active polling for interruption) - maybe.
Yes, it is a similar application implemented in bare metal code directly
on the hardware. The jitter is even smaller with the max being 30ns.
Interesting. W/o having the possibility to have a close look at this
issue, I'd say that this comes from the additional need of hardware
lookups through IRQ rerouting tables. I cannot say.
[snip]
> apic-demo doesn't seem to have much impact on the jitter. How is
cache
> allocated under Jailhouse house? What does a cache allocation of
size 2
> employ?
Cache allocation is only supported on Intel X86, where we have CAT,
cache allocation technology.
I don't know precise details of cat (would have to look them up),
but as
far as I remember, assigning a size of 2 to a non-root-cell means that
2MiB of L3$ are exclusively assigned to the non-root cell. (Look at the
code, I'm not 100% sure).
Why do we do that? Neighbouring noise from other cells.
Just like the non-root cell might "disturb" itself, it's also possible
that Linux, running aside of the non-root cell disturbes it: Imagine
Linux those heavy number crunching while the non-root cell waits for an
IRQ. In that case, Linux will mess up caches of the non-root cell.
CAT prevents full cache pollution by ensuring that there is always a
dedicated, but limited, amount of L3$ for the non-root cell.
Thanks for the explanation, I understand the idea of cache pollution. It
is just that I don't see much impact with or without cache pollution on
the values I get for the jitter. Ideally, the jitter should be higher if
I enable cache pollution and lower when I disable it. But, so far I am
not able to see that.
Also, another question regarding direct interrupt injection, as we saw
that it is enabled by default. Can you tell me how can I disable it?
Again, this is just to see how it will impact the jitter if we have
direct interrupt injection disabled.
There's no actual support for disabling this, it's used when being
available. When Jailhouse tries to query the capability of IRQ remapping
support during startup, you will have to mask out the bits that show
support for it. I guess that should work.
Ralf
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