Re: [PATCH] token based thrashing control

[Song Jiang <sjiang@CS.WM.EDU>]

On Mon, 2 Aug 2004, Rik van Riel wrote:

> On Mon, 2 Aug 2004, Song Jiang wrote:
> 
> > When there is memory competition among multiple processes,
> > Which process grabs the token first is important.
> > A process with its memory demand exceeding the total
> > ram gets the token first and finally has to give it up 
> > due to a time-out would have little performance gain from token,
> > It could also hurt others. Ideally we could make small processes
> > more easily grab the token first and enjoy the benifis from
> > token. That is, we want to protect those that are deserved to be
> > protected. Can we take the rss or other available memory demand
> > information for each process into the consideration of whether 
> > a token should be taken, or given up and how long a token is held.  
> 
> I like this idea.  I'm trying to think of a way to skew
> the "lottery" so small processes get an advantage, but
> the only thing I can come up with is as follows:
> 
> 1) when a process tries to grab the token, it "registers"
>    itself
> 
> 2) a subsequent process can "register" itself to get the
>    token, but only if it has a better score than the
>    process that already has it b
> 
> 3) the score can be calculated based on a few factors,
>    like (a) size of the process (b) time since it last
>    had the token
> 
> 4) a simple formula could be (time / size), giving big
>    processes the token every once in a blue moon and
>    letting small processes have the token all the time

So the score of each registered process, with or without 
token, is calculated periodically. After each calculation,
a registered process with the highest score will take the token.
So a process gives up its token in these 4 cases:
(1) its page fault rate below a threshold (2) its score below
a threshold; (3) it holds a token for too long time (4) it is 
done. 

However, we have to avoid "token thrashing": a token is transfered
among processes too frequently, which could actually create unnecessarily
addtional page faults. So once a process gets the token, we can let
it hold the token for at least a minimal period of time. 
The intention behind the score = time/size is very sound, but
I am not sure how sensitive the performance is to the formula.
We may need to tune it carefully to make it valid.    

Which process will register itself? In my original design,
I allow a process with any major page faults to take the token.
However, I think now we should only allow the processes with their
page fault rate higher than a threshold to register themselves.
In this way we can limit the queue size.



> 
> 5) the token would be grabbed in pretty much the same way
>    we do currently, except the token can be handed to
>    another process instead of the current process, if there
>    is a better candidate registered - all the locking is there
> 
> 6) since there is only one candidate, we won't have any
>    queueing complexities and the algorithm should be just
>    as cheap as it is currently
> 

Do we need to periodically compare the scores of registered processes?
If yes, that would take queueing complexity.

> What do you think ?
> 
> 

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