Comment 5 for bug 1636847

After using the CONFING_SQUASHFS_DECOMP_SINGLE=y option in an experimental kernel the memory usage dropped significantly. The traces below are for -45 and the experimental -46 kernel. The 1 and 4 are the number of CPUs on the system.

A single CPU system used to consumer 131MB per mounted snap, this is now reduced to just 4MB/snap. A four-CPU system has less dramatic improvement where the numbers are 7MB -> 4MB.

This suggests some kind of bug in the CONFIG_SQUASHFS_DECOMP_MULTI_PERCPU option perhaps

+ echo Ubuntu 16.04
Ubuntu 16.04
+ ./analyze.py ubuntu 16.04 4.4.0-45-generic 1 size-1m.squashfs.xz.heavy
# num-mounted extra-memory delta
0: 142.60MB
1: 274.71MB (delta: 132.11MB)
2: 406.55MB (delta: 131.84MB)
3: 538.36MB (delta: 131.81MB)
4: 670.19MB (delta: 131.82MB)
+ ./analyze.py ubuntu 16.04 4.4.0-46-generic 1 size-1m.squashfs.xz.heavy
# num-mounted extra-memory delta
0: 62.28MB
1: 66.99MB (delta: 4.71MB)
2: 71.05MB (delta: 4.07MB)
3: 75.12MB (delta: 4.06MB)
4: 79.19MB (delta: 4.07MB)
+ ./analyze.py ubuntu 16.04 4.4.0-45-generic 4 size-1m.squashfs.xz.heavy
# num-mounted extra-memory delta
0: 235.43MB
1: 242.38MB (delta: 6.96MB)
2: 249.38MB (delta: 7.00MB)
3: 256.45MB (delta: 7.06MB)
4: 263.42MB (delta: 6.97MB)
+ ./analyze.py ubuntu 16.04 4.4.0-46-generic 4 size-1m.squashfs.xz.heavy
# num-mounted extra-memory delta
0: 72.79MB
1: 75.90MB (delta: 3.11MB)
2: 79.96MB (delta: 4.06MB)
3: 83.93MB (delta: 3.97MB)
4: 88.00MB (delta: 4.07MB)