From: Sowmini Varadhan <sowmini.varadhan@xxxxxxxxxx> Date: Thu, 11 Dec 2014 14:45:42 -0500 > 1. lockstat and perf report that iommu->lock is the hot-lock (in a typical > instance, I get about 21M contentions out of 27M acquisitions, 25 us > avg wait time). Even if I fix this issue (see below), I see: The real overhead is unavoidable due to the way the hypervisor access to the IOMMU is implemented in sun4v. If we had direct access to the hardware, we could avoid all of the real overhead in %99 of all IOMMU mappings, as we do for pre-sun4v systems. On sun4u systems, we never flush the IOMMU until we wrap around the end of the IOMMU arena to the beginning in order to service an allocation. Such an optimization is impossible with the hypervisor call interface in sun4v. I've known about this issue for a decade and I do not think there is anything we can really do about this. -- To unsubscribe from this list: send the line "unsubscribe sparclinux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
