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Date: Thu, 15 Dec 2016 11:14:27 -0800
From: Stephen Hemminger <stephen@networkplumber.org>
To: edgar helmut <helmut.edgar100@gmail.com>
Cc: "Hu, Xuekun" <xuekun.hu@intel.com>, "Wiles, Keith"
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Subject: Re: [dpdk-users] Dpdk poor performance on virtual machine
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On Thu, 15 Dec 2016 19:29:54 +0200
edgar helmut <helmut.edgar100@gmail.com> wrote:

> Stephen, this is not the case, it relies on using the transparent hugepages
> which looks like 2M hugepages size.
> Why should be a problem to back 1G pages of the guest to 2M pages at the
> host?
> the transparent hugepages makes the deployment much more flexible.


The IOMMU has a cache (like TLB) which has a limited number of slots.
If using 2M pages then the IOMMU cache misses will cause a performance drop
just like CPU cache misses.  I think Intel had some slides back at IDF 2012
that showed the impact.