linux-kernel - Re: [Intel IOMMU][patch 3/8] Generic hardware support for Intel IOMMU.

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Date:	Wed, 25 Apr 2007 08:53:35 +0200
From:	Andi Kleen <ak@...e.de>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
	akpm@...l.org, gregkh@...e.de, muli@...ibm.com,
	asit.k.mallick@...el.com, suresh.b.siddha@...el.com,
	anil.s.keshavamurthy@...el.com, arjan@...ux.intel.com
Subject: Re: [Intel IOMMU][patch 3/8] Generic hardware support for Intel IOMMU.

On Wednesday 25 A
> > It doesn't seem to enable sg merging? Since you have enough space 
> > that should work.
> We actually have a patch to do sg merge. In my test, it doesn't have any
> performance gain.

It probably depends on the IO device and how slow or fast it is at walking sg lists. 
I saw a small win (few percent gain) some years ago with MPT Fusion chips

> > A GFP_ATOMIC mempool is rather useless. mempool only works if it can block
> > for someone else freeing memory and if it can't do that it's not failsafe.
> > I'm afraid you need to revise the allocation strategy -- best would be
> > to somehow move the memory allocations outside the spinlock paths
> > and preallocate if possible.
> The problem is pci_map_single and friends usually called with interrupt
> disabled or spin locked, so we must use GFP_ATOMIC.

Yes I know, but the mempool still won't work. The design is b0rken.

-Andi
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