Low Memory / Software Emulation Exception / Performance

[Marcus Sundberg]

"Wohlgemuth, Jason" <jason_wohlgemuth at gilbarco.com> 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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