[Xen-devel] [PATCHv4 0/4] evtchn: Improve scalebility

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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)

v4:
- consumer_is_xen() check in __evtchn_close() is only valid when guest
  requested the close.
- Re-add BUG_ON(!port_is_valid()) to free_xen_event_channel().

v3:
- Clear xen_consumer when clearing state.
- Defer freeing struct evtchn's until evtchn_destroy_final().
- Remove redundant d->evtchn test from port_is_valid().
- Use port_is_valid() again.
- Drop event lock from notify_via_xen_event_channel().

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

David


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