On Thu, Nov 15, 2018 at 12:37 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Thu, Nov 15, 2018 at 07:59:20AM -0700, Keith Busch wrote: > > On Thu, Nov 15, 2018 at 05:57:10AM -0800, Matthew Wilcox wrote: > > > On Wed, Nov 14, 2018 at 03:49:14PM -0700, Keith Busch wrote: > > > > Memory-only nodes will often have affinity to a compute node, and > > > > platforms have ways to express that locality relationship. > > > > > > > > A node containing CPUs or other DMA devices that can initiate memory > > > > access are referred to as "memory iniators". A "memory target" is a > > > > node that provides at least one phyiscal address range accessible to a > > > > memory initiator. > > > > > > I think I may be confused here. If there is _no_ link from node X to > > > node Y, does that mean that node X's CPUs cannot access the memory on > > > node Y? In my mind, all nodes can access all memory in the system, > > > just not with uniform bandwidth/latency. > > > > The link is just about which nodes are "local". It's like how nodes have > > a cpulist. Other CPUs not in the node's list can acces that node's memory, > > but the ones in the mask are local, and provide useful optimization hints. > > So ... let's imagine a hypothetical system (I've never seen one built like > this, but it doesn't seem too implausible). Connect four CPU sockets in > a square, each of which has some regular DIMMs attached to it. CPU A is > 0 hops to Memory A, one hop to Memory B and Memory C, and two hops from > Memory D (each CPU only has two "QPI" links). Then maybe there's some > special memory extender device attached on the PCIe bus. Now there's > Memory B1 and B2 that's attached to CPU B and it's local to CPU B, but > not as local as Memory B is ... and we'd probably _prefer_ to allocate > memory for CPU A from Memory B1 than from Memory D. But ... *mumble*, > this seems hard. > > I understand you're trying to reflect what the HMAT table is telling you, > I'm just really fuzzy on who's ultimately consuming this information > and what decisions they're trying to drive from it. The singular "local" is a limitation of the HMAT, but I would expect the Linux translation of "local" would allow for multiple initiators that can achieve some semblance of the "best" performance. Anything less than best is going to have a wide range of variance and will likely devolve to looking at the platform firmware data table directly. The expected 80% case is software wants to be able to ask "which CPUs should I run on to get the best access to this memory?"
