Re: Memory statistics

[michael meeks <michael meeks novell com>]

On Sat, 2006-04-22 at 22:54 +0200, David Turner wrote:
> - due to the way ELF linking works, two identical PLT entries from two
> distinct processes will not necessarily resolve to the same code
> point ! Poof goes our sharing !!

	So - another thing worth mentioning; is that each internal symbol that
is not actually used externally and yet is exported adds  on x86:

+ (potentially) shared mem: (cf. bfd/elfcode.h & include/elf/external.h)
	8bytes of .rel.plt
	16bytes of shared .plt entry
	16bytes of .dynsym
	~<symlen>+1 bytes of .dynstr
+ unshared mem:
	4 bytes of un-sharable GOT entry per process
+ CPU:
	1000's of cycles to fix-up at 1st call + slowing other lookups
	10's of wasted cycles vs. a non-resolved call

	So - it is a worthwhile investment ensuring that platform libraries
export as few symbols as they need to.

	It also (probably) pays to link with -Bsymbolic to reduce .got size: it
might be interesting to re-link gtk+ etc. with -Bsymbolic & compare
before/after .got sizes to see if it's a substantial 'real' win.

	From a memory consumption cost; if you look at:

	readelf -rW foo.so | wc -l

	In an ideal world[1] each time you shrink the reloc count by ~1024
items you save 1 page of memory per app using the lib [ when fully
linked ;-]. However - as this reveals:

	for a in *.so; do num=`readelf -r $a | wc -l`; echo "$num $a"; \
	done | sort -n

	There are only a few libs in GNOME with large numbers of relocs:

859     libgnomeprintui-2-2.so
883     libgnomecupsui-1.0.so
943     libgda-3.so
947     libgsf-1.so
992     libgnomeprint-2-2.so
1036    libbattstat-applet-2.so
1135    libgconf-2.so
1251    liblinphone.so
1474    libeel-2.so
1565    libgdk-x11-2.0.so
1673    libbonoboui-2.so
1680    libgimpwidgets-2.0.so
1692    libgnomeui-2.so
1730    libgnomedb-3.so
1754    libgstreamer-0.8.so
1919    libwpd-1.so
2296    libgnomevfs-2.so
2358    libgtkhtml-3.8.so
2415    libvte.so
2472    libspi.so
3122    libbonobo-2.so
3382    libORBit-2.so
3404    libgtk-x11-2.0.so
3703    libgal-2.4.so
107798  libgucharmap.so

	Not clear to me what libgucharmap is good for ;-)

	By contrast (and I have almost no KDE libs on my machine):

1566    libkdeinit_kio_uiserver.so
3200    libkdeinit_cupsdconf.so
6662    libkmedia2_idl.so
7420    libmcop.so
10732   libsoundserver_idl.so
14626   libartsflow.so
19361   libartsflow_idl.so

	Or worse for OO.o:

22805   libdbu680li.so
26269   libfrm680li.so
26498   libsvt680li.so
27153   liblocaledata_euro.so
31669   libtk680li.so
43096   libsd680li.so
53147   libsc680li.so
62123   libsw680li.so
79609   libsvx680li.so

	;-)

	HTH,

		Michael.

[1] - assuming all these relocs were to adjacant chunks of memory,
seldom true at least for C++.
-- 
 michael meeks novell com  <><, Pseudo Engineer, itinerant idiot