On Mon, Dec 17, 2018 at 2:12 AM Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote: > > On Saturday, December 15, 2018 2:48:30 AM CET Dan Williams wrote: > > Changes since v4: [1] > > * Default the randomization to off and enable it dynamically based on > > the detection of a memory side cache advertised by platform firmware. > > In the case of x86 this enumeration comes from the ACPI HMAT. (Michal > > and Mel) > > * Improve the changelog of the patch that introduces the shuffling to > > clarify the motivation and better explain the tradeoffs. (Michal and > > Mel) > > * Include the required HMAT enabling in the series. > > > > [1]: https://lkml.kernel.org/r/153922180166.838512.8260339805733812034.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > > > > --- > > > > Quote patch 3: > > > > Randomization of the page allocator improves the average utilization of > > a direct-mapped memory-side-cache. Memory side caching is a platform > > capability that Linux has been previously exposed to in HPC > > (high-performance computing) environments on specialty platforms. In > > that instance it was a smaller pool of high-bandwidth-memory relative to > > higher-capacity / lower-bandwidth DRAM. Now, this capability is going to > > be found on general purpose server platforms where DRAM is a cache in > > front of higher latency persistent memory [2]. > > > > Robert offered an explanation of the state of the art of Linux > > interactions with memory-side-caches [3], and I copy it here: > > > > It's been a problem in the HPC space: > > http://www.nersc.gov/research-and-development/knl-cache-mode-performance-coe/ > > > > A kernel module called zonesort is available to try to help: > > https://software.intel.com/en-us/articles/xeon-phi-software > > > > and this abandoned patch series proposed that for the kernel: > > https://lkml.org/lkml/2017/8/23/195 > > > > Dan's patch series doesn't attempt to ensure buffers won't conflict, but > > also reduces the chance that the buffers will. This will make performance > > more consistent, albeit slower than "optimal" (which is near impossible > > to attain in a general-purpose kernel). That's better than forcing > > users to deploy remedies like: > > "To eliminate this gradual degradation, we have added a Stream > > measurement to the Node Health Check that follows each job; > > nodes are rebooted whenever their measured memory bandwidth > > falls below 300 GB/s." > > > > A replacement for zonesort was merged upstream in commit cc9aec03e58f > > "x86/numa_emulation: Introduce uniform split capability". With this > > numa_emulation capability, memory can be split into cache sized > > ("near-memory" sized) numa nodes. A bind operation to such a node, and > > disabling workloads on other nodes, enables full cache performance. > > However, once the workload exceeds the cache size then cache conflicts > > are unavoidable. While HPC environments might be able to tolerate > > time-scheduling of cache sized workloads, for general purpose server > > platforms, the oversubscribed cache case will be the common case. > > > > The worst case scenario is that a server system owner benchmarks a > > workload at boot with an un-contended cache only to see that performance > > degrade over time, even below the average cache performance due to > > excessive conflicts. Randomization clips the peaks and fills in the > > valleys of cache utilization to yield steady average performance. > > > > See patch 3 for more details. > > > > [2]: https://itpeernetwork.intel.com/intel-optane-dc-persistent-memory-operating-modes/ > > [3]: https://lkml.org/lkml/2018/9/22/54 > > Has this hibernation been tested with this series applied? It has not. Is QEMU sufficient? What's your concern?
