m...@bruningsystems.com wrote: > Hi Ben, > Ben Rockwood wrote: >> I'm curious as to why memory statistics seems to be very difficult to be >> accurate about. If you use kstats, mdb ::memstat, and add up VSZ/RSS >> from ps, you get numbers that are different, although close. >> >> Can anyone shed some light on why this is? I'm assumed that ::memstat >> is the most accurate measure and I'm comparing my numbers against it, >> but perhaps it is not the best validation? >> > Have you tried getting the numbers on a crash dump? If you are > doing this on a running system, I would expect the numbers to > fluctuate.
No, I'm not interested in getting so exact that the numbers don't matter because the system isn't running. :) Here is an example: > ::memstat Page Summary Pages MB %Tot ------------ ---------------- ---------------- ---- Kernel 377207 1473 4% Anon 37794 147 0% Exec and libs 5055 19 0% Page cache 12200 47 0% Free (cachelist) 18828 73 0% Free (freelist) 7934179 30992 95% Total 8385263 32754 Physical 8385262 32754 zfs:0:arcstats:size 16504880 <--- 16,504,880 bytes # kstat -p | grep -i system_pages unix:0:system_pages:availrmem 8005191 unix:0:system_pages:class pages unix:0:system_pages:crtime 0 unix:0:system_pages:desfree 65509 unix:0:system_pages:desscan 25 unix:0:system_pages:econtig 4224880640 unix:0:system_pages:fastscan 1681006 unix:0:system_pages:freemem 7954235 unix:0:system_pages:kernelbase 0 unix:0:system_pages:lotsfree 131019 unix:0:system_pages:minfree 32754 unix:0:system_pages:nalloc 25203900 unix:0:system_pages:nalloc_calls 14944 unix:0:system_pages:nfree 23916528 unix:0:system_pages:nfree_calls 9475 unix:0:system_pages:nscan 0 unix:0:system_pages:pagesfree 7954235 <--- 31,816,940 (31071 MB) unix:0:system_pages:pageslocked 380071 unix:0:system_pages:pagestotal 8385262 unix:0:system_pages:physmem 8385263 <--- 33,541,052 (32754 MB) unix:0:system_pages:pp_kernel 379926 <--- 1,519,704k (1484 MB) unix:0:system_pages:slowscan 100 unix:0:system_pages:snaptime 2181598.72132125 So if we look at pages for Kernel, kstat pp_kernel says 379926 but ::memstat says 377207. Of free, kstat pagesfree says 7954235 while ::memstat says 7934179. The numbers are very close (within about 100MB on a system with 32GB of memory) but not exact. If you try to work out "used" memory by, for instance, taking total mem, subtracting free mem, then dividing it between kernel and user you similarly get inconsistence numbers depending on where you get your numbers (::memstat, vs kstat, vs adding up ps RSS numbers). This only really causes trouble because if you try to offer a sysadmin a breakdown of memory it is: 1) Approx within maybe 5% of reality 2) Must be pulled from a single source (kstat?) or the numbers don't even line up as approx And the problem is that this is hard for an end user to swallow... if they attempt to check your math they'll see its not right and discard the data as useless. Thus, perhaps the best take away is that this is why there is no Solaris "memstat" tool for the CLI short of using mdb, and that the more accurate observations are based on how memory is changing (vmstat/mpstat) rather than absolutely what it is at any given point (::memstat). Am I accurate here or out to lunch? benr. _______________________________________________ perf-discuss mailing list perf-discuss@opensolaris.org
