[Xen-users] Network and processing performances

[Andreï V. Fomitchev <fomitchev@xxxxxxxx>]

Hi the XEN-Users list.

I have to evaluate the XEN solution for an industrial project and I am unable to answer/understand some observed questions about network and processing performances.

NB: the tested hardware platform is not optimized for XEN. The processor is an Intel P4 3.2 GHz HT without VMX support. The Dom0 kernel is  2.6.18-6-xen-vserver-686 #1 SMP (from debian etch) and runs on all available VCPUS; DomX (debian etch 2.6.18-4-xen-vserver-686 #1 SMP) uses 1 VCPU.


1) Network performances:
I tried the bridge and route modes but I did not note real difference between these modes (the firewall is "down" in Dom0). So, the performance tests (ssh transfer of 128 MBytes file) have been ran in default bridge mode.
The performances from DomU to Dom0 or to external server and from external server to Dom0 or to DomU are acceptable (ssh transfers at 10 MBytes/s-16 MBytes/s).
The transfer from Dom0 to DomU (5 MBytes/s) and between DomU (1.5 MBytes/s) are slow. In the list archive, I found two possible improvements:
 - increase the sizes of TCP window and of TXQueue
 - disable the CRC checking of TX in DomX ($ ethtool -K <device> tx off)
The first solution does not impact the results however the second increased the transfer between DomX to 12 MBytes/s.
Does it exist another improvement?
What are the QoS impacts when the CRC is not checked? I think nothing because the IP packets do not leave a sure interface and they can not be corrupted. Please, someone can confirm my hypothesis?

2) global DomX performances
Using lmbench and phoronix benchmark suites, I noted that sometimes the DomU performances are better that Dom0. For example,
Lame compression: 146s for DomU and 151s for Dom0
GZip compression: 134.50s for DomU and 350.29s for Dom0(!!!), i.e. DomU is 160% faster that Dom0
LMBench pipe bandwidth communication: 1186 MBytes/s for DomU and 415 MBytes/s for Dom0.
I does not understand the observed behaviours. Please, someone can explain me the differences?

And my last question: the CPU is HT (whose two logical processors) but 'xm vcpu-list' command exhibit that the CPU#1 is not really used by Dom0
$xm vcpu-list
Name                              ID VCPUs   CPU State   Time(s) CPU Affinity
Domain-0                           0     0     0   r--     241.9 any cpu
Domain-0                           0     1     1   -b-       6.3 any cpu
xen-etch                           1     0     1   -b-      53.4 any cpu
xen-etch1                          2     0     1   -b-      41.2 any cpu
Is it XEN+HT-CPU limitation? How works "xm vcpu-pin"? The documentation is not explicit.

Thank you for yours responses.

Best regards,

--
Andreï V. FOMITCHEV
"Ada is PL/I trying to be SmallTalk" -- Codoso diBlini

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