Hi, I am struggling with meaningless values of Committed_AS on a number of Linux systems that I manage or have access to. I do wonder if this is a bug in the kernel (certain versions or configurations) or my lack of understanding even after reading the limited amount of code that seems to deal with the task of counting committed memory (wherever vm_committed_as appears). How can it be that the kernel shows a value for Committed_AS that is _smaller_ than the currently used memory after disregarding cache/buffers? An extreme example is a CentOS 7 NFS server (400 clients) that has MemTotal-MemAvailable of 7431272K, but only Committed_AS of 655532K. This is less than a tenth! MemTotal: 131755776 kB MemFree: 1403468 kB MemAvailable: 124324504 kB Buffers: 82352 kB Cached: 110162312 kB SwapCached: 156 kB Active: 93531636 kB Inactive: 16786948 kB Active(anon): 53472 kB Inactive(anon): 200004 kB Active(file): 93478164 kB Inactive(file): 16586944 kB Unevictable: 0 kB Mlocked: 0 kB SwapTotal: 16777212 kB SwapFree: 16754172 kB Dirty: 16 kB Writeback: 0 kB AnonPages: 73772 kB Mapped: 46068 kB Shmem: 179556 kB Slab: 14604768 kB SReclaimable: 13649928 kB SUnreclaim: 954840 kB KernelStack: 5056 kB PageTables: 5900 kB NFS_Unstable: 0 kB Bounce: 0 kB WritebackTmp: 0 kB CommitLimit: 82655100 kB Committed_AS: 655532 kB VmallocTotal: 34359738367 kB VmallocUsed: 497836 kB VmallocChunk: 34358947836 kB HardwareCorrupted: 0 kB AnonHugePages: 10240 kB CmaTotal: 0 kB CmaFree: 0 kB HugePages_Total: 0 HugePages_Free: 0 HugePages_Rsvd: 0 HugePages_Surp: 0 Hugepagesize: 2048 kB DirectMap4k: 150772 kB DirectMap2M: 6031360 kB DirectMap1G: 130023424 kB How can the committed memory be smaller than the physically used memory? Eeven if it is supposed to be an estimate, it should not be obviously wrong (by an oder of magnitude), should it? I'd think that the kernel does accurately keep track of the committed amount of memory. It for sure looks so with the use of vm_acct_memory(). Am I missing some kernel configuration that intentionally triggers these strangely low values for a reason? I looked at a range of systems and found some with the same inverted relation with Committed_AS, some with numbers that at least do not look obviously wrong. I also built a vanilla 4.14.84 kernel loosely based on the CentOS config and booted a compute node with it. I probably falsely remember that it also showed the issue, but maybe I looked at the wrong system back then. Now this node shows reasonable numbers (0.5G used, 1.8G committed), also on fresh boot where a system with the stock kernel has too low committed memory from the start. For more context, please see https://serverfault.com/questions/942173/ where I included a list of differing systems with the relation of the physical and virtual memory use. Regards, Thomas -- Dr. Thomas Orgis HPC @ Universität Hamburg
