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SubjectRe: [patch] 4GB I/O, cut three
On Wed, 30 May 2001, Jens Axboe wrote:
> On Wed, May 30 2001, Mark Hemment wrote:
> > Hi Jens,
> >
> > I ran this (well, cut-two) on a 4-way box with 4GB of memory and a
> > modified qlogic fibre channel driver with 32disks hanging off it, without
> > any problems. The test used was SpecFS 2.0
> Cool, could you send me the qlogic diff? It's the one-liner can_dma32
> chance I'm interested in, I'm just not sure what driver you used :-)

The qlogic driver is the one from;
I find this much more stable than the one already in the kernel.
It did just need the one-liner change, but as the driver isn't in the
kernel there isn't much point adding it change to your patch. :)

> > I did change the patch so that bounce-pages always come from the NORMAL
> > zone, hence the ZONE_DMA32 zone isn't needed. I avoided the new zone, as
> > I'm not 100% sure the VM is capable of keeping the zones it already has
> > balanced - and adding another one might break the camels back. But as the
> > test box has 4GB, it wasn't bouncing anyway.
> You are right, this is definitely something that needs checking. I
> really want this to work though. Rik, Andrea? Will the balancing handle
> the extra zone?

In theory it should do - ie. there isn't anything to stop it.

With NFS loads, over a ported VxFS filesystem, I do see some problems
between the NORMAL and HIGH zones. Thinking about it, ZONE_DMA32
shouldn't make this any worse.

Rik, Andrea, quick description of a balancing problem;
Consider a VM which is under load (but not stressed), such that
all zone free-page pools are between their MIN and LOW marks, with
pages in the inactive_clean lists.

The NORMAL zone has non-zero page order allocations thrown at
it. This causes __alloc_pages() to reap pages from the NORMAL
inactive_clean list until the required buddy is built. The blind
reaping causes the NORMAL zone to have a large number of free pages
(greater than ->pages_low).

Now, when HIGHMEM allocations come in (for page cache pages), they
skip the HIGH zone and use the NORMAL zone (as it now has plenty
of free pages) - the code at the top of __alloc_pages(), which
checks against ->pages_low.

But the NORMAL zone is usually under more pressure than the HIGH
zone - as many more allocations needed ready-mapped memory. This
causes the page-cache pages from the NORMAL zone to come under
more pressure, and are "re-cycled" quicker than page-cache pages
in the HIGHMEM zone.

OK, we shouldn't be throwing too many non-zero page allocations at
__alloc_pages(), but it does happen.
Also, the problem isn't as bad as it first looks - HIGHMEM page-cache
pages do get "recycled" (reclaimed), but there is a slight imbalance.


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