RPi3B -j4 vs. -j3 main [so: 14] from-scratch buildworld times for my context; buildkernel too; swap space usage and such

RPi3B -j4 vs. -j3 buildworld times for my context:

World built in 120764 seconds, ncpu: 4, make -j4 [So a little under 33 hr 35 min]
World built in 115635 seconds, ncpu: 4, make -j3 [So a little under 32 hr 10 min]
[A delta of a little under 1hr 30min]

So: -j4 buildworld spent more time waiting for its trashing of
the swap space than time it gained from having use of a 4th
core. The trashing is mostly during building of libllvm, libclang,
and liblldb. The RPi3B RAM subsystem can limit the gain from
having more cores active as well.


By contrast . . .

RPi3B -j4 vs. -j3 buildkernel times for my context:

Kernel(s)  GENERIC-NODBG-CA53 built in 7836 seconds, ncpu: 4, make -j4 [So a little under 2 hr 15 min]
Kernel(s)  GENERIC-NODBG-CA53 built in 8723 seconds, ncpu: 4, make -j3 [So a little under 2 hr 30 min]
[A delta of a little under 15 min]

So: -j4 buildkernel spent less time waiting for its trashing of
the swap space than time it gained from having use of a 4th
core. (Not much thrashing occurred.)


And mem/swap usage info for buildworld+buildkernel . . .

Overall -j4 vs -j3 buildworld buildkernel info for my context:

-j4 Mem: . . ., 677688Ki MaxObsActive, 249652Ki MaxObsWired, 950032Ki MaxObs(Act+Wir+Lndry)
-j3 Mem: . . ., 683416Ki MaxObsActive, 315140Ki MaxObsWired, 927424Ki MaxObs(Act+Wir+Lndry)

-j4 Swap: . . ., 1495Mi MaxObsUsed, 2117Mi MaxObs(Act+Lndry+SwapUsed), 2358Mi MaxObs(Act+Wir+Lndry+SwapUsed)
-j3 Swap: . . ., 1178Mi MaxObsUsed, 1811Mi MaxObs(Act+Lndry+SwapUsed), 2049Mi MaxObs(Act+Wir+Lndry+SwapUsed)



FYI for the context:
make[1]: "/usr/main-src/Makefile.inc1" line 326: SYSTEM_COMPILER: Determined that CC=cc matches the source tree.  Not bootstrapping a cross-compiler.
make[1]: "/usr/main-src/Makefile.inc1" line 331: SYSTEM_LINKER: Determined that LD=ld matches the source tree.  Not bootstrapping a cross-linker.


Notes:

Incremental buildworld's would depend on how much rebuilding of
libllvm, libclang, and liblldb would happen to occur.

A system with 2 GiBytes of RAM would have far less trashing of
the swap space. A system with 4 GiBytes of RAM would not thrash
the swap space. The closest comparison I could make with 4
GiBytes of RAM would be the Rock64 doing a from-scratch build.
It is also cortex-a53 based. As I remember, its RAM subsystem
does not limit multiple cores as easily/much. I've no access to
an analogous 2 GiByte context.

===
Mark Millard
marklmi at yahoo.com