From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: [PATCHv2 0/5] evtchn: Improve scalebility
Date: Mon, 15 Jun 2015 16:48:14 +0100
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Cc: Keir Fraser <keir@xen.org>, Tim Deegan <tim@xen.org>, David Vrabel <david.vrabel@citrix.com>, Jan Beulich <jbeulich@suse.com>, Ian Campbell <ian.campbell@citrix.com>
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The per-domain event channel lock limits scalability when many VCPUs
are trying to send interdomain events.  A per-channel lock is
introduced eliminating any lock contention when sending an event.

See this graph for the performance improvements:

  http://xenbits.xen.org/people/dvrabel/evtchn-scalability.png

A different test (using Linux's evtchn device which masks/unmasks
event channels) showed the following lock profile improvements:

Per-domain lock:
(XEN)   lock:    69267976(00000004:19830041), block:    27777407(00000002:3C7C5C96)

Per-event channel lock 
(XEN)   lock:      686530(00000000:076AF5F6), block:        1787(00000000:000B4D22)

Locking removed from evtchn_unmask():
(XEN)   lock:       10769(00000000:00512999), block:          99(00000000:00009491)

v2:
- Use unsigned int for d->valid_evtchns.
- Compare channel pointers in double_evtchn_lock().

David