highmem question

highmem question

Post by Marvin Justic » Sun, 09 Dec 2001 11:10:10

Quote:> That's because of highmem page bouncing when doing I/O. There is indeed
> a solution for this -- 2.5 or 2.4 + block-highmem-all patches will
> happily do I/O directly to any page in your system as long as your
> hardware supports it. I'm sure we're beating w2k with that enabled :-)

Will your patch lead to better performance than the CONFIGH_HIGHMEM=n case?
Unfortunately, W2K with any amount of memory beat Linux with no highmem (see
http://www.uwsg.indiana.edu/hypermail/linux/kernel/0110.3/0375.html ) so my
PHB decided to hold off on Linux for now.

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1. pte-highmem vs UKVA (was: object-based rmap and pte-highmem)

This just just for PTEs ... for which at the moment we have two choices:
1. Stick them in lowmem (fills up the global space too much).
2. Stick them in highmem - too much overhead doing k(un)map_atomic
   as measured by both myself and Andrew.

Using UKVA for PTEs seems to be a better way to implement pte-highmem to me.
If you're walking another processes' pagetables, you just kmap them as now,
but I think this will avoid most of the kmap'ing (if we have space for two
sets of pagetables so we can do a little bit of trickery at fork time).

The pagetables.

Sorry, subject was probably misleading ... I'm just talking about the
PTEs here, not sticking anything to do with rmap into UKVA.

Partially object-based rmap is cool for other reasons, that have little to
do with this. ;-)

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