On Wed, Nov 28, 2012 at 04:47:42PM +0800, Jiang Liu wrote: >Hi all, > Seems it's a great chance to discuss about the memory hotplug feature >within this thread. So I will try to give some high level thoughts about memory >hotplug feature on x86/IA64. Any comments are welcomed! > First of all, I think usability really matters. Ideally, memory hotplug >feature should just work out of box, and we shouldn't expect administrators to >add several extra platform dependent parameters to enable memory hotplug. >But how to enable memory (or CPU/node) hotplug out of box? I think the key point >is to cooperate with BIOS/ACPI/firmware/device management teams. > I still position memory hotplug as an advanced feature for high end >servers and those systems may/should provide some management interfaces to >configure CPU/memory/node hotplug features. The configuration UI may be provided >by BIOS, BMC or centralized system management suite. Once administrator enables >hotplug feature through those management UI, OS should support system device >hotplug out of box. For example, HP SuperDome2 management suite provides interface >to configure a node as floating node(hot-removable). And OpenSolaris supports >CPU/memory hotplug out of box without any extra configurations. So we should >shape interfaces between firmware and OS to better support system device hotplug. > On the other hand, I think there are no commercial available x86/IA64 >platforms with system device hotplug capabilities in the field yet, at least only >limited quantity if any. So backward compatibility is not a big issue for us now. >So I think it's doable to rely on firmware to provide better support for system >device hotplug. > Then what should be enhanced to better support system device hotplug? > >1) ACPI specification should be enhanced to provide a static table to describe >components with hotplug features, so OS could reserve special resources for >hotplug at early boot stages. For example, to reserve enough CPU ids for CPU >hot-add. Currently we guess maximum number of CPUs supported by the platform >by counting CPU entries in APIC table, that's not reliable. > >2) BIOS should implement SRAT, MPST and PMTT tables to better support memory >hotplug. SRAT associates memory ranges with proximity domains with an extra >"hotpluggable" flag. PMTT provides memory device topology information, such >as "socket->memory controller->DIMM". MPST is used for memory power management >and provides a way to associate memory ranges with memory devices in PMTT. >With all information from SRAT, MPST and PMTT, OS could figure out hotplug >memory ranges automatically, so no extra kernel parameters needed. > >3) Enhance ACPICA to provide a method to scan static ACPI tables before >memory subsystem has been initialized because OS need to access SRAT, >MPST and PMTT when initializing memory subsystem. > >4) The last and the most important issue is how to minimize performance >drop caused by memory hotplug. As proposed by this patchset, once we >configure all memory of a NUMA node as movable, it essentially disable >NUMA optimization of kernel memory allocation from that node. According >to experience, that will cause huge performance drop. We have observed >10-30% performance drop with memory hotplug enabled. And on another >OS the average performance drop caused by memory hotplug is about 10%. >If we can't resolve the performance drop, memory hotplug is just a feature >for demo:( With help from hardware, we do have some chances to reduce >performance penalty caused by memory hotplug. > As we know, Linux could migrate movable page, but can't migrate >non-movable pages used by kernel/DMA etc. And the most hard part is how >to deal with those unmovable pages when hot-removing a memory device. >Now hardware has given us a hand with a technology named memory migration, >which could transparently migrate memory between memory devices. There's >no OS visible changes except NUMA topology before and after hardware memory >migration. > And if there are multiple memory devices within a NUMA node, >we could configure some memory devices to host unmovable memory and the >other to host movable memory. With this configuration, there won't be >bigger performance drop because we have preserved all NUMA optimizations. >We also could achieve memory hotplug remove by: >1) Use existing page migration mechanism to reclaim movable pages. >2) For memory devices hosting unmovable pages, we need: >2.1) find a movable memory device on other nodes with enough capacity >and reclaim it. >2.2) use hardware migration technology to migrate unmovable memory to Hi Jiang, Could you give an explanation how hardware migration technology works? Regards, Jaegeuk >the just reclaimed memory device on other nodes. > > I hope we could expect users to adopt memory hotplug technology >with all these implemented. > > Back to this patch, we could rely on the mechanism provided >by it to automatically mark memory ranges as movable with information >from ACPI SRAT/MPST/PMTT tables. So we don't need administrator to >manually configure kernel parameters to enable memory hotplug. > > Again, any comments are welcomed! > >Regards! >Gerry > > >On 2012-11-23 18:44, Tang Chen wrote: >> [What we are doing] >> This patchset provide a boot option for user to specify ZONE_MOVABLE memory >> map for each node in the system. >> >> movablecore_map=nn[KMG]@ss[KMG] >> >> This option make sure memory range from ss to ss+nn is movable memory. >> >> >> [Why we do this] >> If we hot remove a memroy, the memory cannot have kernel memory, >> because Linux cannot migrate kernel memory currently. Therefore, >> we have to guarantee that the hot removed memory has only movable >> memoroy. >> >> Linux has two boot options, kernelcore= and movablecore=, for >> creating movable memory. These boot options can specify the amount >> of memory use as kernel or movable memory. Using them, we can >> create ZONE_MOVABLE which has only movable memory. >> >> But it does not fulfill a requirement of memory hot remove, because >> even if we specify the boot options, movable memory is distributed >> in each node evenly. So when we want to hot remove memory which >> memory range is 0x80000000-0c0000000, we have no way to specify >> the memory as movable memory. >> >> So we proposed a new feature which specifies memory range to use as >> movable memory. >> >> >> [Ways to do this] >> There may be 2 ways to specify movable memory. >> 1. use firmware information >> 2. use boot option >> >> 1. use firmware information >> According to ACPI spec 5.0, SRAT table has memory affinity structure >> and the structure has Hot Pluggable Filed. See "5.2.16.2 Memory >> Affinity Structure". If we use the information, we might be able to >> specify movable memory by firmware. For example, if Hot Pluggable >> Filed is enabled, Linux sets the memory as movable memory. >> >> 2. use boot option >> This is our proposal. New boot option can specify memory range to use >> as movable memory. >> >> >> [How we do this] >> We chose second way, because if we use first way, users cannot change >> memory range to use as movable memory easily. We think if we create >> movable memory, performance regression may occur by NUMA. In this case, >> user can turn off the feature easily if we prepare the boot option. >> And if we prepare the boot optino, the user can select which memory >> to use as movable memory easily. >> >> >> [How to use] >> Specify the following boot option: >> movablecore_map=nn[KMG]@ss[KMG] >> >> That means physical address range from ss to ss+nn will be allocated as >> ZONE_MOVABLE. >> >> And the following points should be considered. >> >> 1) If the range is involved in a single node, then from ss to the end of >> the node will be ZONE_MOVABLE. >> 2) If the range covers two or more nodes, then from ss to the end of >> the node will be ZONE_MOVABLE, and all the other nodes will only >> have ZONE_MOVABLE. >> 3) If no range is in the node, then the node will have no ZONE_MOVABLE >> unless kernelcore or movablecore is specified. >> 4) This option could be specified at most MAX_NUMNODES times. >> 5) If kernelcore or movablecore is also specified, movablecore_map will have >> higher priority to be satisfied. >> 6) This option has no conflict with memmap option. >> >> >> >> Tang Chen (4): >> page_alloc: add movable_memmap kernel parameter >> page_alloc: Introduce zone_movable_limit[] to keep movable limit for >> nodes >> page_alloc: Make movablecore_map has higher priority >> page_alloc: Bootmem limit with movablecore_map >> >> Yasuaki Ishimatsu (1): >> x86: get pg_data_t's memory from other node >> >> Documentation/kernel-parameters.txt | 17 +++ >> arch/x86/mm/numa.c | 11 ++- >> include/linux/memblock.h | 1 + >> include/linux/mm.h | 11 ++ >> mm/memblock.c | 15 +++- >> mm/page_alloc.c | 216 ++++++++++++++++++++++++++++++++++- >> 6 files changed, 263 insertions(+), 8 deletions(-) >> >> >> . >> > > >-- >To unsubscribe, send a message with 'unsubscribe linux-mm' in >the body to majordomo@xxxxxxxxx. For more info on Linux MM, >see: http://www.linux-mm.org/ . >Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
