Messages in this thread | | | Subject | Re: Break 2.4 VM in five easy steps | From | (Eric W. Biederman) | Date | 07 Jun 2001 10:42:55 -0600 |
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LA Walsh <law@sgi.com> writes:
> Now for whatever reason, since 2.4, I consistently use at least > a few Mb of swap -- stands at 5Meg now. Weird -- but I notice things > like nscd running 7 copies that take 72M. Seems like overkill for > a laptop.
So the question becomes why you are seeing an increased swap usage. Currently there are two canidates in the 2.4.x code path.
1) Delayed swap deallocation, when a program exits after it has gone into swap it's swap usage is not freed. Ouch.
2) Increased tenacity of swap caching. In particular in 2.2.x if a page that was in the swap cache was written to the the page in the swap space would be removed. In 2.4.x the location in swap space is retained with the goal of getting more efficient swap-ins.
Neither of the known canidates from increasing the swap load applies when you aren't swapping in the first place. They may aggrevate the usage of swap when you are already swapping but they do not cause swapping themselves. This is why the intial recommendation for increased swap space size was made. If you are swapping we will use more swap.
However what pushes your laptop over the edge into swapping is an entirely different question. And probably what should be solved.
> I think that is the point -- it was supported in 2.2, it is, IMO, > a serious regression that it is not supported in 2.4.
The problem with this general line of arguing is that it lumps a whole bunch of real issues/regressions into one over all perception. Since there are multiple reasons people are seeing problems, they need to be tracked down with specifics.
The swapoff case comes down to dead swap pages in the swap cache. Which greatly increases the number of swap pages slows the system down, but since these pages are trivial to free we don't generate any I/O so don't wait for I/O and thus never enter the scheduler. Making nothing else in the system runnable.
Your case is significantly different. I don't know if you are seeing any issues with swapping at all. With a 5M usage it may simply be totally unused pages being pushed out to the swap space.
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