Add support for smaps_rollup

[amete]

Hi @graeme-a-stewart,

Arguably one of the most expensive operations we do is traversing through smaps to get an accurate measurement of pss. It seems as if the "newer" kernel version have a new file, called /proc/[pid]/smaps_rollup, that provides pre-summed memory information for a given process. It looks to be a very convenient way to simply read the pss measurement rather than reading/summing everything by-hand. We can probably add support for it some time soon (i.e. use it if it's available). What do you think?

Best,
Serhan

[graeme-a-stewart]

Hi @amete - oh, it would be an excellent idea. When we looked the majority of prmon's cycles were going to smaps parsing, so if it can be done more efficiently all the better.

[amete]

Thanks a lot @graeme-a-stewart .

Actually, the "rolled up" version doesn't seem to be a 1-1 replacement since it seems to be missing the Size (which is what we use for vmem), at least on CentOS 8 e.g.:

$ cat /proc/self/smaps_rollup
564007177000-7ffdb8b80000 ---p 00000000 00:00 0                          [rollup]
Rss:                1912 kB
Pss:                 194 kB
Shared_Clean:       1768 kB
Shared_Dirty:          0 kB
Private_Clean:        32 kB
Private_Dirty:       112 kB
Referenced:         1912 kB
Anonymous:           112 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
FilePmdMapped:         0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB

I'm not sure why it's not there since the original proposal clearly includes it. Having said that, we can still get this from /proc/[pid]/statm(status). It'll cost us an extra open/close but considering how large smaps gets for applications such as full reconstruction jobs it might still be beneficial.

[graeme-a-stewart]

At least it would be worth trying - we can implement it and then do a comparison between resources consumed in the two the versions. I suppose it's only available for newer kernels, so we'd need to fall back to the older parser if there's no rollup file.