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Subjectlmbench lat_mmap slowdown with CONFIG_PARAVIRT
Hi,

I'm looking at regressions since 2.6.16, and one is lat_mmap has slowed
down. On further investigation, a large part of this is not due to a
_regression_ as such, but the introduction of CONFIG_PARAVIRT=y.

Now, it is true that lat_mmap is basically a microbenchmark, however it
is exercising the memory mapping and page fault handler paths, so we're
talking about pretty important paths here. So I think it should be of
interest.

I've run the tests on a 2s8c AMD Barcelona system, binding the test to
CPU0, and running 100 times (stddev is a bit hard to bring down, and
my scripts needed 100 runs in order to pick up much smaller changes in
the results -- for CONFIG_PARAVIRT, just a couple of runs should show
up the problem).

Times I believe are in nanoseconds for lmbench, anyway lower is better.

non pv AVG=464.22 STD=5.56
paravirt AVG=502.87 STD=7.36

Nearly 10% performance drop here, which is quite a bit... hopefully people
are testing the speed of their PV implementations against non-PV bare
metal :)


CPU: AMD64 family10, speed 2000 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit mask of 0x00 (No unit mask) count 10000
samples % symbol name
BASE
49749 32.6336 read_tsc
10151 6.6587 __up_read
7363 4.8299 unmap_vmas
7072 4.6390 mnt_drop_write
4107 2.6941 do_page_fault
4090 2.6829 rb_get_reader_page
3601 2.3621 apic_timer_interrupt
3537 2.3202 set_page_dirty
3435 2.2532 mnt_want_write
3302 2.1660 default_idle
2990 1.9613 idle_cpu
2904 1.9049 file_update_time
2789 1.8295 set_page_dirty_balance
2712 1.7790 retint_swapgs
2455 1.6104 __do_fault
2231 1.4635 release_pages
1989 1.3047 rb_buffer_peek
1895 1.2431 ring_buffer_consume
1572 1.0312 handle_mm_fault
1554 1.0194 put_page
1461 0.9584 sync_buffer
1196 0.7845 clear_page_c
1145 0.7511 rb_advance_reader
1144 0.7504 hweight64
1084 0.7111 getnstimeofday
1076 0.7058 __set_page_dirty_no_writeback
1020 0.6691 mark_page_accessed
751 0.4926 tick_do_update_jiffies64
CONFIG_PARAVIRT
8924 7.8849 native_safe_halt
8823 7.7957 native_read_tsc
6201 5.4790 default_spin_lock_flags
5806 5.1300 unmap_vmas
3996 3.5307 handle_mm_fault
3954 3.4936 rb_get_reader_page
3752 3.3151 __do_fault
2908 2.5694 getnstimeofday
2303 2.0348 apic_timer_interrupt
2183 1.9288 find_busiest_group
2057 1.8175 do_page_fault
2057 1.8175 hweight64
2017 1.7821 set_page_dirty
1926 1.7017 get_next_timer_interrupt
1781 1.5736 release_pages
1702 1.5038 native_pte_val
1620 1.4314 native_sched_clock
1588 1.4031 rebalance_domains
1558 1.3766 run_timer_softirq
1531 1.3527 __down_read_trylock
1505 1.3298 native_pmd_val
1445 1.2767 find_get_page
1369 1.2096 find_next_bit
1356 1.1981 __ticket_spin_lock
1225 1.0824 shmem_getpage
1169 1.0329 radix_tree_lookup_slot
1166 1.0302 vm_normal_page
987 0.8721 scheduler_tick


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