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library: prefer /proc/stat before /proc/status, <pids>

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Long ago, in a galaxy far away (when top was in charge
of library FILL flgs) /proc/status was to be preferred
over /proc/stat if a field could be satisfied by both.

This was done to avoid costly 64-bit math emulation in
a 32-bit application due to 'unsigned long long' data.

Well it's time to acknowledge the prevalence of 64-bit
platforms. And in such an environment the cost picture
has shifted significantly. It now costs 14 times (wow)
as much to access /proc/status compared to /proc/stat.

In other words, even with '%llu' variables, a sscanf()
call in stat2proc() beats the pants off that home brew
gperf based hashing employed by the status2proc() guy.
In fact, status2proc incurs higher costs than found in
the most expensive aspect of top's forest view option.

Here's a gprof extract to illustrate the costs. It was
produced with an rcfile requiring fields from both the
/proc/stat & /proc/status pseudo files (among others).
There were 5000 iterations in each of 4 separate gprof
runs subsequently merged into 1 gmon.sum for analysis.

  %      self              self
 time   seconds    calls  us/call  name
 -----  -------  -------  -------  -----------
 28.65     4.10  4689423     0.87  status2proc
 26.14     3.74    40000    93.50  forest_adds
 ...
 01.96     0.28  4689427     0.06  stat2proc

[ since forest_adds is recursive, the calls value is ]
[ the non-recursive #, its 'call graph' shows totals ]

Anyway, now that such cost is known this patch becomes
what is euphemistically known as the usual no-brainer.

[ jeeze, was it really this long between profilings? ]

Signed-off-by: Jim Warner <james.warner@comcast.net>
This commit is contained in:
Jim Warner 2016-08-21 00:00:00 -05:00 committed by Craig Small
parent 4e4debda9b
commit e9c101edcb
1 changed files with 3 additions and 3 deletions
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6
proc/pids.c
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@ -339,7 +339,7 @@ srtDECL(noop) {
// ___ Controlling Table ||||||||||||||||||||||||||||||||||||||||||||||||||||||
// from either 'stat' or 'status' (preferred)
#define f_either PROC_SPARE_1
#define f_either PROC_SPARE_1 // either status or stat (favor stat)
#define f_grp PROC_FILLGRP
#define f_lxc PROC_FILL_LXC
#define f_ns PROC_FILLNS
@ -913,8 +913,8 @@ static inline void pids_libflags_set (
info->history_yes |= Item_table[e].needhist;
}
if (info->oldflags & f_either) {
if (!(info->oldflags & f_stat))
info->oldflags |= f_status;
if (!(info->oldflags & (f_stat | f_status)))
info->oldflags |= f_stat;
}
return;
} // end: pids_libflags_set