[uml-devel] slow execution in uml?

["Young K" <young_uml@hotmail.com>]

hi, folks,

i was comparing uml network performance with genuine linux. the experiment i 
did was to insert time-stamping code in network stack, and see how slower 
uml is. i was sending 32-byte udp packets to another machine every 10 msec, 
and used rdtsc to measure the elapsed cpu cycles between sendto() and 
nf_hook_slow() (which is called inside ip_build_xmit()).

what i found was that it took around 29,000 cycles from sendto() to 
nf_hook_slow() in uml whereas only 2,000 cycles in genuine linux. it looks 
like a big difference. but i think it's purely code execution without any 
I/O operation between sendto() to nf_hook_slow(). then, why is it so slow in 
uml? is there any possible factors that make uml slower? (more cache 
misses?)

i used 2.4.22 for guest os(tt mode) and 2.4.25 for host os, but numbers were 
pretty much the same when i used other versions of 2.4 kernels for both 
guest and host. the computer i used for test was P4 2.2 GHz single processor 
with 512MB, FYI.

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