Xen devel - Dec 2013 - [PATCH net-next v2 0/9] xen-netback: TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy

A long known problem of the upstream netback implementation that on the TX
path (from guest to Dom0) it copies the whole packet from guest memory into
Dom0. That simply became a bottleneck with 10Gb NICs, and generally
it''s a
huge perfomance penalty. The classic kernel version of netback used grant
mapping, and to get notified when the page can be unmapped, it used page
destructors. Unfortunately that destructor is not an upstreamable solution.
Ian Campbell''s skb fragment destructor patch series [1] tried to solve
this
problem, however it seems to be very invasive on the network stack''s
code,
and therefore haven''t progressed very well.
This patch series use SKBTX_DEV_ZEROCOPY flags to tell the stack it needs to
know when the skb is freed up. That is the way KVM solved the same problem,
and based on my initial tests it can do the same for us. Avoiding the extra
copy boosted up TX throughput from 6.8 Gbps to 7.9 (I used a slower
Interlagos box, both Dom0 and guest on upstream kernel, on the same NUMA node,
running iperf 2.0.5, and the remote end was a bare metal box on the same 10Gb
switch)
Based on my investigations the packet get only copied if it is delivered to
Dom0 stack, which is due to this [2] patch. That''s a bit unfortunate,
but
luckily it doesn''t cause a major regression for this usecase. In the
future
we should try to eliminate that copy somehow.
There are a few spinoff tasks which will be addressed in separate patches:
- grant copy the header directly instead of map and memcpy. This should help
  us avoiding TLB flushing
- use something else than ballooned pages
- fix grant map to use page->index properly
I will run some more extensive tests, but some basic XenRT tests were already
passed with good results.
I''ve tried to broke it down to smaller patches, with mixed results, so
I
welcome suggestions on that part as well:
1: Introduce TX grant map definitions
2: Change TX path from grant copy to mapping
3: Remove old TX grant copy definitons and fix indentations
4: Change RX path for mapped SKB fragments
5: Add stat counters for zerocopy
6: Handle guests with too many frags
7: Add stat counters for frag_list skbs
8: Timeout packets in RX path
9: Aggregate TX unmap operations

v2: I''ve fixed some smaller things, see the individual patches.
I''ve added a
few new stat counters, and handling the important use case when an older guest
sends lots of slots. Instead of delayed copy now we timeout packets on the RX
path, based on the assumption that otherwise packets should get stucked
anywhere else. Finally some unmap batching to avoid too much TLB flush

[1] http://lwn.net/Articles/491522/
[2] https://lkml.org/lkml/2012/7/20/363

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>

On 2013/12/13 7:48, Zoltan Kiss wrote:
> A long known problem of the upstream netback implementation that on the TX
> path (from guest to Dom0) it copies the whole packet from guest memory into
> Dom0. That simply became a bottleneck with 10Gb NICs, and generally
it''s a
> huge perfomance penalty. The classic kernel version of netback used grant
> mapping, and to get notified when the page can be unmapped, it used page
> destructors. Unfortunately that destructor is not an upstreamable solution.
> Ian Campbell''s skb fragment destructor patch series [1] tried to
solve this
> problem, however it seems to be very invasive on the network
stack''s code,
> and therefore haven''t progressed very well.
> This patch series use SKBTX_DEV_ZEROCOPY flags to tell the stack it needs
to
> know when the skb is freed up. That is the way KVM solved the same problem,
> and based on my initial tests it can do the same for us. Avoiding the extra
> copy boosted up TX throughput from 6.8 Gbps to 7.9 (I used a slower
> Interlagos box, both Dom0 and guest on upstream kernel, on the same NUMA
node,
> running iperf 2.0.5, and the remote end was a bare metal box on the same
10Gb
> switch)
Sounds good.
Is the TX throughput gotten between one vm and one bare metal box? or 
between multiple vms and bare metal? Do you have any test results with 
netperf?

Thanks
Annie
> Based on my investigations the packet get only copied if it is delivered to
> Dom0 stack, which is due to this [2] patch. That''s a bit
unfortunate, but
> luckily it doesn''t cause a major regression for this usecase. In
the future
> we should try to eliminate that copy somehow.
> There are a few spinoff tasks which will be addressed in separate patches:
> - grant copy the header directly instead of map and memcpy. This should
help
>    us avoiding TLB flushing
> - use something else than ballooned pages
> - fix grant map to use page->index properly
> I will run some more extensive tests, but some basic XenRT tests were
already
> passed with good results.
> I''ve tried to broke it down to smaller patches, with mixed
results, so I
> welcome suggestions on that part as well:
> 1: Introduce TX grant map definitions
> 2: Change TX path from grant copy to mapping
> 3: Remove old TX grant copy definitons and fix indentations
> 4: Change RX path for mapped SKB fragments
> 5: Add stat counters for zerocopy
> 6: Handle guests with too many frags
> 7: Add stat counters for frag_list skbs
> 8: Timeout packets in RX path
> 9: Aggregate TX unmap operations
>
> v2: I''ve fixed some smaller things, see the individual patches.
I''ve added a
> few new stat counters, and handling the important use case when an older
guest
> sends lots of slots. Instead of delayed copy now we timeout packets on the
RX
> path, based on the assumption that otherwise packets should get stucked
> anywhere else. Finally some unmap batching to avoid too much TLB flush
>
> [1] http://lwn.net/Articles/491522/
> [2] https://lkml.org/lkml/2012/7/20/363
>
> Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
>
>
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