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Re: [Xen-devel] [PATCH 0/4] Implement persistent grant in xen-netfront/netback





On 2012-11-16 17:57, Ian Campbell wrote:
On Thu, 2012-11-15 at 07:03 +0000, Annie Li wrote:
This patch implements persistent grants for xen-netfront/netback.
Hang on a sec. It has just occurred to me that netfront/netback in the
current mainline kernels don't currently use grant maps at all, they use
grant copy on both the tx and rx paths.

Ah, this patch is based on v3.4-rc3.
Current mainline kernel does not pass the netperf/netserver case. As I mentioned earlier, I hit BUG_ON with your debug patch too when testing mainline kernel with netperf/netserver.
This is interesting, I should have check the latest code.


The supposed benefit of persistent grants is to avoid the TLB shootdowns
on grant unmap, but in the current code there should be exactly zero of
those.

Is there any performance document about current grant copy code in mainline kernel?


If I understand correctly this patch goes from using grant copy
operations to persistently mapping frames and then using memcpy on those
buffers to copy in/out to local buffers. I'm finding it hard to think of
a reason why this should perform any better, do you have a theory which
explains it?

This patch is aiming to fix spin lock issue of grant operations, it comes out to avoid possible grant operations(including grant map and copy).

(my best theory is that it has a beneficial impact on where
the cache locality of the data, but netperf doesn't typically actually
access the data so I'm not sure why that would matter)

Also AIUI this is also doing persistent grants for both Tx and Rx
directions?

Yes.


For guest Rx does this mean it now copies twice, in dom0 from the DMA
buffer to the guest provided buffer and then again in the guest from the
granted buffer to a normal one?

Yes.


For guest Tx how do you handle the lifecycle of the grant mapped pages
which are being sent up into the dom0 network stack? Or are you also now
copying twice in this case? (i.e. guest copies into a granted buffer and
dom0 copies out into a local buffer?)

Copy twice: guest copies into a granted buffer and dom0 copies out into a local buffer.


Did you do measurement of the Tx and Rx cases independently?

No.

Do you know
that they both benefit from this change (rather than for example an
improvement in one direction masking a regression in the other).

On theory, this implementation avoid spinlock issue of grant operation, so they should both benefit from it.

Were
the numbers you previously posted in one particular direction or did you
measure both?

One particular direction, one runs as server, the other runs as client.

Thanks
Annie

Ian.


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