[coyotos-dev] Some things to consider

Haplo starfirex at comcast.net
Thu Nov 30 02:44:13 CST 2006


I've been researching swarm intelligence and other applications of  
stochastic optimization, and came across a few things.

First is LURCH. LURCH is a debugger/software verification tool which  
uses random monte-carlo searching on an application to find linked  
clusters of problematic code. Because of the way it operates, LURCH  
can very quickly (minutes as opposed to hours or days for  
deterministic approaches) find the vast majority of compromising code  
in your application. Also, its resource usage does not grow very much  
even for very large projects. While unsuited for formal verification,  
it can greatly speed the process of correcting your current code and  
adding new code, while making it much safer to add code to the trunk  
build. Since you no longer have to rely on by-hand analysis of your  
code, you no longer simply have to triple check and pray that you got  
it right. Finally, when it comes down to formal verifications for  
release builds, after the long process of verification is over,  
you'll have much less code to dig through and fix since you'd have  
been doing it incrementally the entire time. LURCH should allow  
speeding the development of coyotos and allow a stable (and fully  
verified) release build to be produced more rapidly.

Information about LURCH:
http://en.wikipedia.org/wiki/LURCH

Further info about the principles used:
http://en.wikipedia.org/wiki/Global_optimization

Now, the main goal of these techniques is to quickly locate global  
optima (which takes great deals of time using conventional methods  
due to the large numbers of variables involved and the time required  
to absolutely graph them all). While these methods may not find the  
absolute global optimum, they can produce values which are very very  
close and do so very quickly. One such example would be distributing  
bandwidth on a communications network. It occurred to me that this  
sort of problem is very similar to scheduling, only with scheduling  
you can know almost all of the variables for a given application (and  
its dependents) in need of scheduling. As such, you could fairly  
easily create a process to scan an application (like LURCH does) to  
find its connections and determine optimal or near optimal scheduling  
for its statistically most significant communications (IPC) paths, as  
well as a fallback scheduling for other things, and scheduling for  
worst-case performance situations (ie maxed system load).  
Furthermore, you could also use it to generate global tables to  
ensure near-optimum global performance at all times while still  
maintaining good to near optimal local performance for individual  
applications.

Comparison with software feedback scheduling:
-Software feedback scheduling requires unauthorized channels to  
provide the basis for the scheduling. Stochastic pre-optimized  
scheduling does not (it reads through the normal IPC/capability  
sequences most often used by the applications).

-Software feedback scheduling has a degree of hysteresis in  
compensating for new or varying loads. Furthermore, it requires  
constant guess-and-check adjustment which can take considerable  
processing time itself in a situation with highly variable load (or  
else increases its hysteresis which reduces its effectiveness). SPO  
would require one sum of time when installing a new application for  
building the application's scheduling table and adjusting the global  
table (these would be similar to capability tables). After that,  
scheduling would be reactive based on the tables. Hysteresis would be  
minimal and scheduling would simply mean reading the tables based on  
the current request by the application and the state of the system  
(no guessing).

-Software feedback also has to deal with new situations as they come.  
SPO would be redundant, so it's run a few times read many. There are  
essentially no 'new situations' to SPO, it already basically 'knows'  
what to do in given situations. It doesn't perform quite as well  
under statistically unlikely conditions (code that's only run  
rarely), but the failsafe scheduling is still reasonably optimal or  
at least very acceptable.

-Software feedback works towards local optima. That is, it maximizes  
the connections between two applications. As such, under worst-case  
performance (processor is overburdened) it will perform very badly.  
SPO pre-optimizes for such situations and can provide decent to very  
good system wide performance even under the heaviest loads. That is,  
you shouldn't get any beachballing or frozen GUIs just because the  
system is working on a large problem, but the large problem will  
still get ample resources to complete on time (or in a reasonable  
time if there is no specific timeout constraint).

-NICE value type scheduling when compared to software feedback is  
grossly inaccurate, somewhat like a manual transmission only you only  
change gears once every few thousand miles. Software feedback,  
however, is still considerably less accurate than SPO overall,  
somewhat like a continuously variable transmission vs a CVT that  
already knows where the hills are.

Although its usage would be quite limited for embedded applications  
where the system is very small (that is, scheduling is simple), I  
should imagine that if it works, it would make coyotos perform  
significantly better than monokernels in the majority of situations.  
This is something I would really like to see in a formally verified  
capabilities based KOS. That would make it easy to popularize  
coyotos, which could change the general opinion on microkernels.


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