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Statistics provided through PCS SDK. As we have only async interface in SDK we need to be subscribed to statistics in order to get it. Trivial solution on every stat request to subscribe, wait event and then unsubscribe will lead to significant delays in case of a number of successive requests, as the event will be delivered on next PCS server notify cycle. On the other hand we don't want to keep unnesessary subscribtion. So we take an hibrid solution to subcsribe on first request and then keep a subscription while requests are active. We populate cache of statistics on subscribtion events and use this cache to serve libvirts requests. * Cache details. Cache is just handle to last arrived event, we call this cache as if this handle is valid it is used to serve synchronous statistics requests. We use number of successive events count to detect that user lost interest to statistics. We reset this count to 0 on every request. If more than PARALLELS_STATISTICS_DROP_COUNT successive events arrive we unsubscribe. Special value of -1 of this counter is used to differentiate between subscribed/unsubscribed state to protect from delayed events. Values of PARALLELS_STATISTICS_DROP_COUNT and PARALLELS_STATISTICS_TIMEOUT are just drop-ins, choosen without special consideration. * Thread safety issues Use parallelsDomObjFromDomainRef in parallelsDomainBlockStats as we could wait on domain lock down on stack in prlsdkGetStatsParam and if we won't keep reference we could get dangling pointer on return from wait. Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@parallels.com>
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>