Linux-kernel mailing list archive 1998-00,: Re: 2.1.76, nfs client, and memory fragmentation

# Re: 2.1.76, nfs client, and memory fragmentation

Alan Cox (alan@lxorguk.ukuu.org.uk)
Fri, 2 Jan 1998 00:37:11 +0000 (GMT)

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> > No, no reverse page tables. The data structures to keep them up-to-date
> > worries me (it would essentially involve doubling all the page tables).
>
> One word: Eh?
>
> Based on what I did with Nachos it would take only one page table entry *
> per page of physical memory; (alternately, the order-0 free bitmap [cw]ould

How do you handle shared pages - you need more than one pointer in this case.

> Right now I see it as a cheap and elegant solution to the fragmentation
> problem which involves low-coefficient O(1) work to maintain and one kbyte
> of storage per megabyte of physical memory. My 32M machine can certainly
> trade 32k it can't use when it needs it, for 16k (and larger) fragments
> on demand...

Put it this way - Im willing to drop 32K of memory for the right results
and test the patch if you do it. To start with I can get that 32K back
simply by putting Andy's select/poll patches into my kernel and non inlining
the copy_to/from_user which btw my benchmarks say speeds up the machine

Alan

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