From mboxrd@z Thu Jan  1 00:00:00 1970
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: Load increase after memory upgrade (part2)
Date: Mon, 5 Dec 2011 22:26:21 -0500
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To: Carsten Schiers <carsten@schiers.de>
Cc: Konrad Rzeszutek Wilk <konrad@darnok.org>, xen-devel <xen-devel@lists.xensource.com>, "lersek@redhat.com" <lersek@redhat.com>, Ian Campbell <Ian.Campbell@citrix.com>, "zhenzhong.duan@oracle.com" <zhenzhong.duan@oracle.com>
List-Id: xen-devel@lists.xenproject.org

On Sun, Dec 04, 2011 at 01:09:28PM +0100, Carsten Schiers wrote:
> Here with two cards enabled and creating a bit "work" by watching TV with one oft hem:
> 
> [   23.842720] Starting SWIOTLB debug thread.
> [   23.842750] swiotlb_start_thread: Go!
> [   23.842838] xen_swiotlb_start_thread: Go!
> [   28.841451] 0 [budget_av 0000:00:01.0] bounce: from:435596(slow:0)to:0 map:658 unmap:0 sync:435596
> [   28.841592] SWIOTLB is 4% full
> [   33.840147] 0 [budget_av 0000:00:01.0] bounce: from:127652(slow:0)to:0 map:0 unmap:0 sync:127652
> [   33.840283] SWIOTLB is 4% full
> [   33.844222] 0 budget_av 0000:00:01.0 alloc coherent: 8, free: 0
> [   38.840227] 0 [budget_av 0000:00:01.0] bounce: from:128310(slow:0)to:0 map:0 unmap:0 sync:128310

Whoa. Yes. You are definitly using the bounce buffer :-)

Now it is time to look at why the drive is not using those coherent ones - it
looks to allocate just eight of them but does not use them.. Unless it is
using them _and_ bouncing them (which would be odd).

And BTW, you can lower your 'swiotlb=XX' value.  The 4% is how much you
are using of the default size.

I should find out_why_ the old Xen kernels do not use the bounce buffer
so much...