Unix & Linux Asked on October 31, 2021
My server runs out of memory even though there is swap available.
Why?
I can reproduce it this way:
eat_20GB_RAM() {
perl -e '$a="c"x10000000000;print "OKn";sleep 10000';
}
export -f eat_20GB_RAM
parallel -j0 eat_20GB_RAM ::: {1..25} &
When that stabilizes (i.e. all processes reach sleep) I run a few more:
parallel --delay 5 -j0 eat_20GB_RAM ::: {1..25} &
When that stabilizes (i.e. all processes reach sleep) around 800 GB RAM/swap is used:
$ free -m
total used free shared buff/cache available
Mem: 515966 440676 74514 1 775 73392
Swap: 1256720 341124 915596
When I run a few more:
parallel --delay 15 -j0 eat_20GB_RAM ::: {1..50} &
I start to get:
Out of memory!
even though there is clearly swap available.
$ free
total used free shared buff/cache available
Mem: 528349276 518336524 7675784 14128 2336968 7316984
Swap: 1286882284 1017746244 269136040
Why?
$ cat /proc/meminfo
MemTotal: 528349276 kB
MemFree: 7647352 kB
MemAvailable: 7281164 kB
Buffers: 70616 kB
Cached: 1503044 kB
SwapCached: 10404 kB
Active: 476833404 kB
Inactive: 20837620 kB
Active(anon): 476445828 kB
Inactive(anon): 19673864 kB
Active(file): 387576 kB
Inactive(file): 1163756 kB
Unevictable: 18776 kB
Mlocked: 18776 kB
SwapTotal: 1286882284 kB
SwapFree: 269134804 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 496106244 kB
Mapped: 190524 kB
Shmem: 14128 kB
KReclaimable: 753204 kB
Slab: 15772584 kB
SReclaimable: 753204 kB
SUnreclaim: 15019380 kB
KernelStack: 46640 kB
PageTables: 3081488 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 1551056920 kB
Committed_AS: 1549560424 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 1682132 kB
VmallocChunk: 0 kB
Percpu: 202752 kB
HardwareCorrupted: 0 kB
AnonHugePages: 0 kB
ShmemHugePages: 0 kB
ShmemPmdMapped: 0 kB
FileHugePages: 0 kB
FilePmdMapped: 0 kB
CmaTotal: 0 kB
CmaFree: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
Hugetlb: 0 kB
DirectMap4k: 12251620 kB
DirectMap2M: 522496000 kB
DirectMap1G: 3145728 kB
In /proc/meminfo
you find:
CommitLimit: 1551056920 kB
Committed_AS: 1549560424 kB
So you are at the commit limit.
If you have disabled overcommiting of memory (to avoid the OOM-killer) by:
echo 2 > /proc/sys/vm/overcommit_memory
Then the commit limit is computed as:
2 - Don't overcommit. The total address space commit
for the system is not permitted to exceed swap + a
configurable amount (default is 50%) of physical RAM.
Depending on the amount you use, in most situations
this means a process will not be killed while accessing
pages but will receive errors on memory allocation as
appropriate.
(From: https://www.kernel.org/doc/Documentation/vm/overcommit-accounting)
You can use the full memory by:
echo 100 > /proc/sys/vm/overcommit_ratio
Then you will get out-of-memory when physical RAM and swap is all reserved.
The name overcommit_ratio
is in this case a bit misleading: You are not overcommitting anything.
Even with this setup you may see out-of-memory before swap is exhausted. malloc.c:
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <unistd.h>
void main(int argc, char **argv) {
long bytes, sleep_sec;
if(argc != 3) {
printf("Usage: malloc bytes sleep_secn");
exit(1);
}
sscanf(argv[1],"%ld",&bytes);
sscanf(argv[2],"%ld",&sleep_sec);
printf("Bytes: %ld Sleep: %ldn",bytes,sleep_sec);
if(malloc(bytes)) {
sleep(sleep_sec);
} else {
printf("Out of memoryn");
exit(1);
}
}
Compile as:
gcc -o malloc malloc.c
Run as (reserve 1 GB for 10 seconds):
./malloc 1073741824 10
If you run this you may see OOM even though there is swap free:
# Plenty of ram+swap free before we start
$ free -m
total used free shared buff/cache available
Mem: 515966 2824 512361 16 780 511234
Swap: 1256720 0 1256720
# Reserve 1.8 TB
$ ./malloc 1800000000000 100 &
Bytes: 1800000000000 Sleep: 100
# It looks as if there is plenty of ram+swap free
$ free -m
total used free shared buff/cache available
Mem: 515966 2824 512361 16 780 511234
Swap: 1256720 0 1256720
# But there isn't: It is all reserved (just not used yet)
$ cat /proc/meminfo |grep omm
CommitLimit: 1815231560 kB
Committed_AS: 1761680484 kB
# Thus this fails (as you would expect)
$ ./malloc 180000000000 100
Bytes: 180000000000 Sleep: 100
Out of memory
So while free
in practice often will do The Right Thing, looking at CommitLimit and Committed_AS seems to be more bullet-proof.
Answered by Ole Tange on October 31, 2021
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