On 07.04.2021 08:05, Bharata B Rao wrote: > On Wed, Apr 07, 2021 at 08:28:07AM +1000, Dave Chinner wrote: >> On Mon, Apr 05, 2021 at 11:18:48AM +0530, Bharata B Rao wrote: >>> Hi, >>> >>> When running 10000 (more-or-less-empty-)containers on a bare-metal Power9 >>> server(160 CPUs, 2 NUMA nodes, 256G memory), it is seen that memory >>> consumption increases quite a lot (around 172G) when the containers are >>> running. Most of it comes from slab (149G) and within slab, the majority of >>> it comes from kmalloc-32 cache (102G) >>> >>> The major allocator of kmalloc-32 slab cache happens to be the list_head >>> allocations of list_lru_one list. These lists are created whenever a >>> FS mount happens. Specially two such lists are registered by alloc_super(), >>> one for dentry and another for inode shrinker list. And these lists >>> are created for all possible NUMA nodes and for all given memcgs >>> (memcg_nr_cache_ids to be particular) >>> >>> If, >>> >>> A = Nr allocation request per mount: 2 (one for dentry and inode list) >>> B = Nr NUMA possible nodes >>> C = memcg_nr_cache_ids >>> D = size of each kmalloc-32 object: 32 bytes, >>> >>> then for every mount, the amount of memory consumed by kmalloc-32 slab >>> cache for list_lru creation is A*B*C*D bytes. >>> >>> Following factors contribute to the excessive allocations: >>> >>> - Lists are created for possible NUMA nodes. >>> - memcg_nr_cache_ids grows in bulk (see memcg_alloc_cache_id() and additional >>> list_lrus are created when it grows. Thus we end up creating list_lru_one >>> list_heads even for those memcgs which are yet to be created. >>> For example, when 10000 memcgs are created, memcg_nr_cache_ids reach >>> a value of 12286. >> >> So, by your numbers, we have 2 * 2 * 12286 * 32 = 1.5MB per mount. >> >> So for that to make up 100GB of RAM, you must have somewhere over >> 500,000 mounted superblocks on the machine? >> >> That implies 50+ unique mounted superblocks per container, which >> seems like an awful lot. > > Here is how the calculation turns out to be in my setup: > > Number of possible NUMA nodes = 2 > Number of mounts per container = 7 (Check below to see which are these) > Number of list creation requests per mount = 2 > Number of containers = 10000 > memcg_nr_cache_ids for 10k containers = 12286 Luckily, we have "+1" in memcg_nr_cache_ids formula: size = 2 * (id + 1). In case of we only multiplied it, you would have to had memcg_nr_cache_ids=20000. Maybe, we need change that formula to increase memcg_nr_cache_ids more accurate for further growths of containers number. Say, size = id < 2000 ? 2 * (id + 1) : id + 2000 > size of kmalloc-32 = 32 byes > > 2*7*2*10000*12286*32 = 110082560000 bytes = 102.5G > >> >>> - When a memcg goes offline, the list elements are drained to the parent >>> memcg, but the list_head entry remains. >>> - The lists are destroyed only when the FS is unmounted. So list_heads >>> for non-existing memcgs remain and continue to contribute to the >>> kmalloc-32 allocation. This is presumably done for performance >>> reason as they get reused when new memcgs are created, but they end up >>> consuming slab memory until then. >>> - In case of containers, a few file systems get mounted and are specific >>> to the container namespace and hence to a particular memcg, but we >>> end up creating lists for all the memcgs. >>> As an example, if 7 FS mounts are done for every container and when >>> 10k containers are created, we end up creating 2*7*12286 list_lru_one >>> lists for each NUMA node. It appears that no elements will get added >>> to other than 2*7=14 of them in the case of containers. >> >> Yeah, at first glance this doesn't strike me as a problem with the >> list_lru structure, it smells more like a problem resulting from a >> huge number of superblock instantiations on the machine. Which, >> probably, mostly have no significant need for anything other than a >> single memcg awareness? >> >> Can you post a typical /proc/self/mounts output from one of these >> idle/empty containers so we can see exactly how many mounts and >> their type are being instantiated in each container? > > Tracing type->name in alloc_super() lists these 7 types for > every container. > > 3-2691 [041] .... 222.761041: alloc_super: fstype: mqueue > 3-2692 [072] .... 222.812187: alloc_super: fstype: proc > 3-2692 [072] .... 222.812261: alloc_super: fstype: tmpfs > 3-2692 [072] .... 222.812329: alloc_super: fstype: devpts > 3-2692 [072] .... 222.812392: alloc_super: fstype: tmpfs > 3-2692 [072] .... 222.813102: alloc_super: fstype: tmpfs > 3-2692 [072] .... 222.813159: alloc_super: fstype: tmpfs > >> >>> One straight forward way to prevent this excessive list_lru_one >>> allocations is to limit the list_lru_one creation only to the >>> relevant memcg. However I don't see an easy way to figure out >>> that relevant memcg from FS mount path (alloc_super()) >> >> Superblocks have to support an unknown number of memcgs after they >> have been mounted. bind mounts, child memcgs, etc, all mean that we >> can't just have a static, single mount time memcg instantiation. >> >>> As an alternative approach, I have this below hack that does lazy >>> list_lru creation. The memcg-specific list is created and initialized >>> only when there is a request to add an element to that particular >>> list. Though I am not sure about the full impact of this change >>> on the owners of the lists and also the performance impact of this, >>> the overall savings look good. >> >> Avoiding memory allocation in list_lru_add() was one of the main >> reasons for up-front static allocation of memcg lists. We cannot do >> memory allocation while callers are holding multiple spinlocks in >> core system algorithms (e.g. dentry_kill -> retain_dentry -> >> d_lru_add -> list_lru_add), let alone while holding an internal >> spinlock. >> >> Putting a GFP_ATOMIC allocation inside 3-4 nested spinlocks in a >> path we know might have memory demand in the *hundreds of GB* range >> gets an NACK from me. It's a great idea, but it's just not a >> feasible, robust solution as proposed. Work out how to put the >> memory allocation outside all the locks (including caller locks) and >> it might be ok, but that's messy. > > Ok, I see the problem and it looks like hard to get allocations > outside of those locks. > >> >> Another approach may be to identify filesystem types that do not >> need memcg awareness and feed that into alloc_super() to set/clear >> the SHRINKER_MEMCG_AWARE flag. This could be based on fstype - most >> virtual filesystems that expose system information do not really >> need full memcg awareness because they are generally only visible to >> a single memcg instance... > > This however seems like a feasible approach, let me check on this. > > Regards, > Bharata. >
