Christian Borntraeger [off-list ref] wrote on 20/08/2014 
11:41:32 AM:

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Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 

MB/sec).

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Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running 

netperf

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(4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits 

was reduced by

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31%.
The same experiment with 3 vms running filebench showed similar 

numbers.

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Signed-off-by: Razya Ladelsky <redacted>

Gave it a quick try on s390/kvm. As expected it makes no difference 
for big streaming workload like iperf.
uperf with a 1-1 round robin got indeed faster by about 30%.
The high CPU consumption is something that bothers me though, as 
virtualized systems tend to be full.

Thanks for confirming the results!
The best way to use this patch would be along with a shared vhost thread 
for multiple
devices/vms, as described in:
http://domino.research.ibm.com/library/cyberdig.nsf/1e4115aea78b6e7c85256b360066f0d4/479e3578ed05bfac85257b4200427735!OpenDocument
This work assumes having a dedicated I/O core where the vhost thread 
serves multiple vms, which 
makes the high cpu utilization less of a concern. 

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+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of 

virtqueue when rate of events is at least this number per jiffy. If 
0, never start polling.");

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+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of 

virtqueue after this many jiffies of no work.");

This seems ridicoudly high. Even one jiffie is an eternity, so 
setting it to 1 as a default would reduce the CPU overhead for most 

cases.

If we dont have a packet in one millisecond, we can surely go back 
to the kick approach, I think.

Christian

Good point, will reduce it and recheck.
Thank you,
Razya