Re: [00/17] Large Blocksize Support V3

From: Nick Piggin
Date: Fri Apr 27 2007 - 22:28:13 EST

Christoph Hellwig wrote:
On Fri, Apr 27, 2007 at 10:25:44PM +1000, Nick Piggin wrote:

Linus's favourite jokes about powerpc mmu being crippled forever, aside ;)

Different mmu. The desktop 32bit mmu Linus refered to has almost nothing
in common with the mmu on 64bit systems.

Well I wasn't trying to make a point there so it isn't a big deal... but
he has known to say the 64-bit hash table is insane or broken. If he's
since recanted, I'd be interested to read the post :)

Right this could help but it is not addressing the basic requirement for
devices that need large contiguuos chunks of memory for I/O.

Did you read the last paragraph? Or anything Andrew's been writing?

"After that, I'd find it amusing if HBAs worth thousands of $ have
trouble looking up sglists at the relatively glacial pace that IO
requires, and/or can't spare a few more K for reasonable sglist
sizes, but if that is really the case, then we could use iommus
and/or just attempt to put physically contiguous pages in pagecache,
rather than require it."

Real highend HBAs don't have that problem. But for example aacraid
which is very common on mid-end servers is a _lot_ faster when it
gets continous memory. Some benchmark was 10 or more percent faster
on windows due to this.

And that wasn't due to the 128 sg limit?

I guess 10% isn't a small amount. Though it would be nice to have
before/after numbers for Linux. And, like Andrew was saying, we could
just _attempt_ to put contiguous pages in pagecache rather than
_require_ it. Which is still robust under fragmentation, and benefits
everyone, not just files with a large pagecache size.

SUSE Labs, Novell Inc.
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