Il 13/03/2014 11:54, David Vrabel ha scritto:
On 12/03/14 18:54, Waiman Long wrote:
Locking is always an issue in a virtualized environment as the virtual
CPU that is waiting on a lock may get scheduled out and hence block
any progress in lock acquisition even when the lock has been freed.
One solution to this problem is to allow unfair lock in a
para-virtualized environment. In this case, a new lock acquirer can
come and steal the lock if the next-in-line CPU to get the lock is
scheduled out. Unfair lock in a native environment is generally not a
good idea as there is a possibility of lock starvation for a heavily
contended lock.
I do not think this is a good idea -- the problems with unfair locks are
worse in a virtualized guest. If a waiting VCPU deschedules and has to
be kicked to grab a lock then it is very likely to lose a race with
another running VCPU trying to take a lock (since it takes time for the
VCPU to be rescheduled).
Actually, I think the unfair version should be automatically selected if
running on a hypervisor. Per-hypervisor pvops can choose to enable the
fair one.
Lock unfairness may be particularly evident on a virtualized guest when
the host is overcommitted, but problems with fair locks are even worse.
In fact, RHEL/CentOS 6 already uses unfair locks if
X86_FEATURE_HYPERVISOR is set. The patch was rejected upstream in favor
of pv ticketlocks, but pv ticketlocks do not cover all hypervisors so
perhaps we could revisit that choice.
Measurements were done by Gleb for two guests running 2.6.32 with 16
vcpus each, on a 16-core system. One guest ran with unfair locks, one
guest ran with fair locks. Two kernel compilations ("time make -j 16
all") were started at the same time on both guests, and times were as
follows:
unfair: fair:
real 13m34.674s real 19m35.827s
user 96m2.638s user 102m38.665s
sys 56m14.991s sys 158m22.470s
real 13m3.768s real 19m4.375s
user 95m34.509s user 111m9.903s
sys 53m40.550s sys 141m59.370s
Actually, interpreting the numbers shows an even worse slowdown.
Compilation took ~6.5 minutes in a guest when the host was not
overcommitted, and with unfair locks everything scaled just fine.
Ticketlocks fell completely apart; during the first 13 minutes they were
allotted 16*6.5=104 minutes of CPU time, and they spent almost all of it
spinning in the kernel (102 minutes in the first run). They did perhaps
30 seconds worth of work because, as soon as the unfair-lock guest
finished and the host was no longer overcommitted, compilation finished
in 6 minutes.
So that's approximately 12x slowdown from using non-pv fair locks (vs.
unfair locks) on a 200%-overcommitted host.
Paolo
With the unfair locking activated on bare metal 4-socket Westmere-EX
box, the execution times (in ms) of a spinlock micro-benchmark were
as follows:
# of Ticket Fair Unfair
tasks lock queue lock queue lock
------ ------- ---------- ----------
1 135 135 137
2 1045 1120 747
3 1827 2345 1084
4 2689 2934 1438
5 3736 3658 1722
6 4942 4434 2092
7 6304 5176 2245
8 7736 5955 2388
Are these figures with or without the later PV support patches?
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