David Lang a écrit :
I have a fork-heavy workload (a proxy that forks per connection, I know
it's not the most efficiant design) and I discovered a 2x performance
difference between a static and dynamicly linked version of the same
program (2200 connections/sec vs 4700 connections/sec)
I know that there is overhead on program startup, but didn't expect to
find it on a fork with no exec. If I has been asked I would have guessed
that the static version would have been slower due to the need to mark
more memory as COW.
what is it that costs so much with dynamic libraries on a fork/clone?
man ld.so
LD_BIND_NOW
If set to non-empty string, causes the dynamic linker to resolve
all symbols at program startup instead of deferring function
call resolval to the point when they are first referenced.
If you do :
export LD_BIND_NOW=1
before starting your dynamicaly linked version, do you get different numbers ?
If some symbols are resolved dynamically after your forks(), the dynamic
linker has to dirty some parts of memory and each child gets its own copy of
modified pages. The cpu cost is not factorized, and memory needs are larger,
so cpu caches are less efficient.
With LD_BIND_NOW=1, the initial exec of your programm will be a litle bit
longer, but in the end you win.
You may see effect of immediate binding with ldd command :
Its -r option asks to do the full binding :
# time ldd ./groff
libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0xf7ea0000)
libm.so.6 => /lib/tls/libm.so.6 (0xf7e7e000)
libgcc_s.so.1 => /lib/libgcc_s.so.1 (0xf7e73000)
libc.so.6 => /lib/tls/libc.so.6 (0x42000000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0xf7f5d000)
0.00user 0.00system 0:00.00elapsed 80%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+696minor)pagefaults 0swaps
# time ldd -r ./groff
libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0xf7e8f000)
libm.so.6 => /lib/tls/libm.so.6 (0xf7e6d000)
libgcc_s.so.1 => /lib/libgcc_s.so.1 (0xf7e62000)
libc.so.6 => /lib/tls/libc.so.6 (0x42000000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0xf7f4c000)
0.00user 0.00system 0:00.00elapsed 50%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+777minor)pagefaults 0swaps
You can see 777 pagefaults instead of 696 on this example.
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