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

Mon Aug 21 12:15:21 EDT 2017

Hi Pau,

I personally don't think we need this in asyncio.  While the function has a relatively low overhead, it's still an overhead, it's a couple more syscalls on each loop iteration, and it's a bit of a technical debt.

The bar for adding new functionality to asyncio is very high, and I don't see a good rationale for adding this function.  Is it for debugging purposes?  Or for profiling live applications?  If it's the latter, then there are so many other things we want to see, and some of them are protocol-specific.

If we want to add some tracing/profiling functions there should be a way to disable them, otherwise the performance of event loops like uvloop will degrade, and I'm not sure that all of its users want to pay a price for something they won't ever be using.  All of this just adds to the complexity.

OTOH, event loops in asyncio are pluggable.  You can just subclass the asyncio event loop and add your method. asyncio code base is very stable, so you won't need to fix your code frequently.

Thanks,
Yury

On Aug 20, 2017, 7:27 PM -0400, Pau Freixes <pfreixes at gmail.com>, wrote:
> Hi,
>
> I have a second implementation of the load function [1], the previous
> one was too much naive. The main changes compared with the previous
> one are:
>
> 1) Uses a decay function instead of a vector of samples.
> 2) The load stands for the percentage of CPU used, giving a global
> view of how many CPU resources are still unused.
> 3) The update is done at each _LOAD_FREQ - default 1 second - without
> using a scheduler callback
> 4) Many corner cases fixed
>
> The performance impact introduced in the default loop implemented with
> Python is approx 3% running a trivial program that does not have an
> application overhead [2]. Therefore, in real applications with at
> least some footprint introduced by the application, this performance
> impact should be negligible.
>
> As an example of how the load method can be used, the following code
> [3] runs the loop using different ratios of coroutines per second,
> where each coroutine has a CPU impact of 0.02. Having a maximum
> throughput expected of 50 coroutines per second. In this example, the
> coroutine asks first for the load of the system before start consuming
> the CPU, if the load is higher than 0.9 the coroutine leaves doing
> nothing.
>
> The following snippet shows the execution output :
>
> Load reached for 10.0 coros/seq: 0.20594804872227002, abandoned 0/100
> reseting load....
> Load reached for 20.0 coros/seq: 0.40599215789994814, abandoned 0/200
> reseting load....
> Load reached for 40.0 coros/seq: 0.8055964270483202, abandoned 0/400
> reseting load....
> Load reached for 80.0 coros/seq: 0.9390106831339007, abandoned 450/800
>
> The program runs as was said different levels of throughput printing
> at the final of each one the load of the system reached, and the
> coroutines that were abandoned vs the overall. As can be seen, once
> the last test begins to run and the load of the system reaches the
> 0.90 it is able to reduce the pressure at the application level, in
> that case just leaving doing nothing but in other environments doing
> the proper fallback. As you can notice the load values seem to be
> aligned with the values expected, having in mind that the maximum
> throughput reached would be 50 coroutines per second.
>
> The current implementation tries to return the load taking into
> account the global CPU resources, therefore other processes impacting
> on the use of the same CPU used by the loop should be considered. It
> would give to the developer a reliable metric that can be used in
> environments where the CPU is shared by other processes.
>
> What is missing? Investigate how difficult will be implemented this
> feature in libuv [4] to make it available in uvloop. Give more
> information about the difference between this implementation vs the
> toobusy one [5]. Test performance impact in real applications.
>
> I will like to get more feedback from you. And, if you believe that
> this implementation has some chances to be part of CPython repo which
> would be the next steps that I should make.
>
>
> [1] https://github.com/pfreixes/cpython/commit/ac07fef5af51746c7311494f21b0f067c772a2bf
> [2] https://gist.github.com/pfreixes/233fd8c6a6ec82f2cde4688a2976bf2d
> [3] https://gist.github.com/pfreixes/fd26c36391b33056b7efd525e4690aef
> [4] http://docs.libuv.org/en/v1.x/
> [5] https://github.com/lloyd/node-toobusy
>
> On Sun, Aug 13, 2017 at 12:54 PM, Pau Freixes <pfreixes at gmail.com> wrote:
> > > 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.
> >
> > The implementation proposed wants to expose the load of the loop.
> > Having a direct metric that comes from the loop instead of using an
> > external metric such as CPU, load average u others.
> >
> > Yes, the load average uses a decay function based on the length of the
> > run queue for those processes that are using or waiting for a CPU,
> > this gives us extra information about how overloaded is our system. If
> > you compare it with the CPU load.
> >
> > In the case presented, the load of the loop is something equivalent
> > with the load of the CPU and it does not have the ability to inform
> > you about how much overloaded is your loop once reached the 100%.
> >
> > >
> > > 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.
> >
> > Well, I partially disagree with this. The load definition has its
> > equivalent in computing with other metrics that have a close range,
> > such as the CPU one. I've never had the intention to align the load of
> > the loop with the load average, I've just used the concept as an
> > example of the metric that might be used to check how loaded is your
> > system.
> >
> > >
> > > 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 the 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.
> >
> > Here the key for me, something where I should have based my rationale.
> > How good is the way presented to measure a load of your asynchronous
> > system compared with the toobusy one? what can we achieve with this
> > metric?
> >
> > I will work on that as the base of my rationale for the change
> > proposed. Then, once if the rationale is accepted the implementation
> > is peanuts :)
> >
> > --
> > --pau
>
>
>
> --
> --pau
> _______________________________________________
> Async-sig mailing list
> Async-sig at python.org
> https://mail.python.org/mailman/listinfo/async-sig
> Code of Conduct: https://www.python.org/psf/codeofconduct/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/async-sig/attachments/20170821/ea8c669c/attachment.html>