Re: "[KSWAPD] linux-2.4.8-pre4, <kswapd profile test results>

[Marcelo Tosatti]

On Mon, 6 Aug 2001, Eric Taylor wrote:

> Kswapd has been seen to get busy for minutes. I used the kernel
> profiler with a minor mod, to analyse what kswapd is doing during this
> time. I think that everyone agrees that it is probably running too
> often, but I wanted to see what it was doing when my system froze.
> 
> Eric
> please cc at [e-mail]
> 
> 
> 
> 
> 
> My test case is simple, esentially the below loop: 
> 
> =================================================
> 32-bit int array = malloc(1.2 gigs of mem);
> for(I=0; I < 1.2gig/4 ; I++){
>    array[I] = I;
> }
> =================================================
> 
> 
> 
> =================================================
> Findings (with approximate numbers):
> =================================================
> 
> The time spent in kswapd is almost entirely in scan_swap_map (normally inlined in swapfile.c).
> Note that it is called like this:
> 
>                         swap_device_lock(p);
>                         offset = scan_swap_map(p, count);
>                         swap_device_unlock(p);
> 
> What does swap_device_lock lock, could it explain why nothing else runs while kswapd is scanning?

We hold the pagetable lock for the mm when scanning it.

> Over a typical run, scan_swap_map was called 461k (461,000) times.
> Only .6k times did it find a completely empty cluster (approx 1 /
> 1000). 175k times it found a free page, 286k times it did not find a
> free page. In this test, the clustering code is not finding many
> clusters, but seems to be doing a lot of work. The swapfile cluster
> value is 256 (perhaps this should become a tuning variable).

That code was written a long time ago.

I started to write a new clustering scheme for swapping 4h ago.  

I'll post code as soon as I get it stable.