Re: [Async-sig] Feedback, loop.load() function

[Nathaniel Smith]

It looks like your "load average" is computing something very different
than the traditional Unix "load average". If I'm reading right, yours is a
measure of what percentage of the time the loop spent sleeping waiting for
I/O, taken over the last 60 ticks of a 1 second timer (so generally
slightly longer than 60 seconds). The traditional Unix load average is an
exponentially weighted moving average of the length of the run queue.

Is one of those definitions better for your goal of detecting when to shed
load? I don't know. But calling them the same thing is pretty confusing
:-). The Unix version also has the nice property that it can actually go
above 1; yours doesn't distinguish between a service whose load is at
exactly 100% of capacity and barely keeping up, versus one that's at 200%
of capacity and melting down. But for load shedding maybe you always want
your tripwire to be below that anyway.

More broadly we might ask what's the best possible metric for this purpose
– how do we judge? A nice thing about the JavaScript library you mention is
that scheduling delay is a real thing that directly impacts quality of
service – it's more of an "end to end" measure in a sense. Of course, if
you really want an end to end measure you can do things like instrument
your actual logic, see how fast you're replying to http requests or
whatever, which is even more valid but creates complications because some
requests are supposed to take longer than others, etc. I don't know which
design goals are important for real operations.

On Aug 6, 2017 3:57 PM, "Pau Freixes"  wrote:

> Hi guys,
>
> I would appreciate any feedback about the idea of implementing a new
> load function to ask about how saturated is your reactor.
>
> I have a proof of concept [1] of how the load function might be
> implemented in the Asyncio python loop.
>
> The idea is to provide a method that can be used to ask about the load
> of the reactor in a specific time, this implementation returns the
> load taking into account the last 60 seconds but it can easily return
> the 5m and 15minutes ones u others.
>
> This method can help services built on to of Asyncio to implement back
> pressure mechanisms that take into account a metric coming from the
> loop, instead of inferring the load using other metrics provided by
> external agents such as the CPU, load average u others.
>
> Nowadays exists some alternatives for other languages that address
> this situation using the lag of a scheduler callback, produced by
> saturated reactors. The most known implementation is toobusy [2] a
> nodejs implementation.
>
> IMHO the solution provided by tobusy has a strong dependency with the
> hardware needing to tune the maximum lag allowed in terms of
> milliseconds [3]. in the POF presented the user can use an exact value
> meaning the percentage of the load, perhaps 0.9
>
> Any comment would be appreciated.
>
> [1] https://github.com/pfreixes/cpython/commit/
> 5fef3cae043abd62165ce40b181286e18f5fb19c
> [2] https://www.npmjs.com/package/toobusy
> [3] https://www.npmjs.com/package/toobusy#tunable-parameters
> --
> --pau
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Re: [Async-sig] Feedback, loop.load() function

[Chris Jerdonek]

On Sun, Aug 6, 2017 at 3:57 PM, Pau Freixes  wrote:
> Hi guys,
>
> I would appreciate any feedback about the idea of implementing a new
> load function to ask about how saturated is your reactor.

Hi,

Would it be possible for you to rephrase what you've done in terms of
asyncio terminology? From what I can tell, "reactor" isn't a term used
in the asyncio docs or code base. It might also improve the
readability of your asyncio patch to use asyncio terminology in the
code comments, doc strings, etc.

--Chris


>
> I have a proof of concept [1] of how the load function might be
> implemented in the Asyncio python loop.
>
> The idea is to provide a method that can be used to ask about the load
> of the reactor in a specific time, this implementation returns the
> load taking into account the last 60 seconds but it can easily return
> the 5m and 15minutes ones u others.
>
> This method can help services built on to of Asyncio to implement back
> pressure mechanisms that take into account a metric coming from the
> loop, instead of inferring the load using other metrics provided by
> external agents such as the CPU, load average u others.
>
> Nowadays exists some alternatives for other languages that address
> this situation using the lag of a scheduler callback, produced by
> saturated reactors. The most known implementation is toobusy [2] a
> nodejs implementation.
>
> IMHO the solution provided by tobusy has a strong dependency with the
> hardware needing to tune the maximum lag allowed in terms of
> milliseconds [3]. in the POF presented the user can use an exact value
> meaning the percentage of the load, perhaps 0.9
>
> Any comment would be appreciated.
>
> [1] 
> https://github.com/pfreixes/cpython/commit/5fef3cae043abd62165ce40b181286e18f5fb19c
> [2] https://www.npmjs.com/package/toobusy
> [3] https://www.npmjs.com/package/toobusy#tunable-parameters
> --
> --pau
> ___
> Async-sig mailing list
> Async-sig@python.org
> https://mail.python.org/mailman/listinfo/async-sig
> Code of Conduct: https://www.python.org/psf/codeofconduct/
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