Re: Low Memory / Software Emulation Exception / Performance

"Wohlgemuth, Jason" [off-list ref] writes:

Just a few more questions.  After applying the head.S patch our software
emulation exceptions have gone away, although, I intend to go back and trip
the exception with a logic analyzer attached to verify everything with our
hardware engineer.  However, this patch seems to induce ultra-slow
performance in areas where we map physical memory down to the user-level
with /dev/mem, my guess is that it has something to do with this:

Tring to map in RAM via /dev/mem is equivalent to mapping /dev/zero -
you'll get anonymous private pages (which are cached). Don't ask me
why it is done like that... Trying to map in memory above RAM via
/dev/mem will work as intended, and will always give you caching
inhibited pages.

The dirty page fix should not alter this behaviour afaikt.

Which is it that you are doing?

Specifically, the part regarding _PAGE_WRITETHRU being redefined to
_PAGE_NO_CACHE, is this a correct assumption?

Yes, but _PAGE_WRITETHRU is not used by anything in the standard
kernel so it shouldn't make any difference.

 Is anyone else running into performance related issues with this
patch applied?

I haven't noted anything like that, but on the other hand we have
no apps mapping /dev/mem.

//Marcus
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