Re: HIGHMEM4G config for 1GB RAM on desktop?

[Andrew Morton <akpm@osdl.org>]

Jens Axboe <axboe@suse.de> wrote:
>
> > > > -#define __PAGE_OFFSET		(0xC0000000)
>  > > > +#define __PAGE_OFFSET		(0xB0000000)
>  > > >  #else
>  > > > -#define __PAGE_OFFSET		(0xC0000000UL)
>  > > > +#define __PAGE_OFFSET		(0xB0000000UL)
>  > > >  #endif
>  > >
>  > > Yup precisely. I agree that there probably isn't a whole lot of
>  > > performance hit on a 1GB, it just seems silly that we need highmem on
>  > > such a standard memory configuration these days. Especially when just
>  > > moving the offset slightly removes that need.
>  > 
>  > As a desktop user with 1024MB ram I agree that HIMEM has a silly threshold and 
>  > should not need to be enabled in this case. Its becoming common, especially 
>  > with dual channel memory systems to use 2x512MB sticks. On a hunch I bet 
>  > 2x512 is more common that 1x512 and 1x256 so why not merge this up? Who would 
>  > we submit this patch to?
> 
>  One way would be to ask Andrew what he thinks?

The 896M/128M split has a bit of a problem now each zone has its own LRU:
the size of the highmem zone is less than the amount of memory which is
described by the default /proc/sys/vm/dirty_ratio.  So it is easy to
completely fill highmem with dirty pages.  This causes a fairly large
amount of writeback via vmscan.c's writepage().  This causes poor I/O
submission patterns.  This causes a simple large, linear `dd' write to run
at only 50-70% of disk bandwidth.  (This was 6-12 months ago - it might be
a bit better now)

But I seem to be the only person who has noticed this yet ;) A workaround
is to decrease dirty_ratio and dirty_background_ratio.

Decreasing PAGE_OFFSET as above is attractive, but I believe 0xc0000000 is
part of the ABI, and although we know (from the 4g/4g and other such
patches) that everything will work OK, I wonder if it's really worth doing,
especially as it's a compile-time thing.

But hey, if someone can identify specific benefits from it then perhaps
sneaking in a config option, or maintaining an external patch would be
worthwhile.