Re: VM: question

Rik van Riel (H.H.vanRiel@fys.ruu.nl)
Tue, 14 Apr 1998 17:16:16 +0200 (MET DST)


On 5 Apr 1998, Eric W. Biederman wrote:

> I'm suffering from memory fragmentation slow downs, and my machine
> just has to be up for a while before memory gets sufficiently
> fragmented to cause trouble :(
>
> Anyone want to explain to me really slowly why we try to keep huge
> chunks of contiguous memory?

DMA, 8k NFS fragments, etc...

> And why if that is important why we don't implement a relocating
> defragmentation algorithm in the kernel? On the assumption that I
> could pause the kernel for a moment, it would be probably faster to do
> that on demand, then the current mess!

Even better, make sure that fragmentation doesn't occur very
often by freeing pages on demand before we use a free page
from a big free area.

> There is a defragmentation algorithm that runs in O(mem_size) time
> with two passes over memory, and needs no extra memory.

But it does need huge amounts of CPU time... memcpy() isn't
exactly cheap :(

Rik.
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