Add PSS (proportional set size), Swap and SwapPSS calculation on Linux

[ntninja]

I did not write this code and took a patch for this from https://github.com/linvinus/htop-mod. Please tell me about any improvements required.

While I can attest PSS to be a big improvement over RSS, there are some pain points about this:

• /proc/<pid>/smaps is only readable by the (effective?) user of the given process. ~Since PSS calculation requires all memory mapping tables to be available for accurate calculation, this makes the PSS value somewhat useless htop runs as root.~ Actually this is not true making PSS data somewhat usable, but incomplete for users other than the current user, unless you're root. :slightly_frowning_face:
• Greatly improved on Linux 4.14+ thanks to @himikof's suggestion! (Crunching the smaps data noticeably slows down htop – far from unusable, but slower.)

Original code was written by Craig M. Brandenburg for htop 1.0.2 Many performance improvements by GitHub user linvinus, ported to htop 2.0.2

[himikof]

Maybe the code could be extended to try smaps_rollup if available (in kernel since 4.14)? The format is exactly the same, just less data to parse.

[ntninja]

@himikof: Thanks for the suggestion! I've implemented your suggestion and it does give quite a performance boost. Still not as quite snappy as when not reading any smaps file, but it stops the UI from feeling somewhat sluggish while PSS reporting is enabled.

[hishamhm]

Thanks for the patch! I still need to take some time to give it a spin, but on first glance my initial comment is that we'll probably not want to enable PSS by default, given the performance concerns.

[ntninja]

I agree one that sentiment unfortunately. The original patch had it in by default, but given the speed on pre-4.14 kernels and the root requirement (that RSS and friends doesn't have) for complete reporting it is really a sane default currently. Let me know of any other things you notice and I'll apply the requested updates once there is more to do.

[ntninja]

@hishamhm: I have already explained why this is not a problem in the comment above. Mind considering a merge again?

[ntninja]

@hishamhm: Ping

[natoscott]

Merged here: htop-dev/htop@f9625ca