[Libunwind-devel] Re: [patch] Fix for race in dwarf_find_save_locs

[Paul Pluzhnikov]

On Tue, Nov 24, 2009 at 10:54 AM, Arun Sharma <address@hidden> wrote:

> Executing apply_reg_state with the lock held is a problem only for
> UNW_CACHE_GLOBAL.

With lock *not* held. Correct.

> How does the performance of UNW_CACHE_PER_THREAD compare
> in your tests?

In google3 tests? I don't have a good way to measure that.

I have tried to set UNW_CACHE_PER_THREAD as I was debugging this
race, but that caused crashes I didn't understand; perhaps I should
revisit that.

Hmm, I don't see how it could work at all in current code :(

Doesn't using UNW_CACHE_PER_THREAD require that unw_local_addr_space
in x86*/Ginit.c be made a per-thread variable? Otherwise, all threads
will share that global, but will not lock it.


For Gperf-simple, there is no discernible difference (data below),
but it only uses one thread.

> I'm inclined to apply the more conservative fix #1 until we have more data
> on the cost of the memcpy vs using UNW_CACHE_PER_THREAD.

My concern with fix#1 is that it reduces concurrency in a hot function
(apply_reg_state) -- we have CPUs to burn!


Data from running tests/Gperf-simple:


--- current ---

unw_getcontext : cold avg=  150.204 nsec, warm avg=   38.147 nsec
unw_init_local : cold avg=  259.876 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1848.312 min= 1341.956 avg= 1413.348 nsec
global cache    : unw_step : 1st=  390.552 min=  131.698 avg=  180.194 nsec
per-thread cache: unw_step : 1st=  390.552 min=  131.698 avg=  171.771 nsec

unw_getcontext : cold avg=  159.740 nsec, warm avg=   47.684 nsec
unw_init_local : cold avg=  278.950 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1941.408 min= 1341.956 avg= 1424.901 nsec
global cache    : unw_step : 1st=  342.869 min=  131.698 avg=  167.376 nsec
per-thread cache: unw_step : 1st=  304.268 min=  131.698 avg=  159.256 nsec

unw_getcontext : cold avg=  147.820 nsec, warm avg=   40.531 nsec
unw_init_local : cold avg=  259.876 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1855.124 min= 1360.121 avg= 1414.406 nsec
global cache    : unw_step : 1st=  429.153 min=  170.299 avg=  180.939 nsec
per-thread cache: unw_step : 1st=  304.268 min=  131.698 avg=  173.349 nsec

unw_getcontext : cold avg=  138.283 nsec, warm avg=   40.531 nsec
unw_init_local : cold avg=  259.876 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1961.844 min= 1341.956 avg= 1367.185 nsec
global cache    : unw_step : 1st=  390.552 min=  131.698 avg=  140.364 nsec
per-thread cache: unw_step : 1st=  267.937 min=  131.698 avg=  134.995 nsec



--- fix #1 ---

unw_getcontext : cold avg=  138.283 nsec, warm avg=   50.068 nsec
unw_init_local : cold avg=  278.950 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1866.477 min= 1341.956 avg= 1409.588 nsec
global cache    : unw_step : 1st=  361.034 min=  131.698 avg=  152.169 nsec
per-thread cache: unw_step : 1st=  417.800 min=  131.698 avg=  164.150 nsec

unw_getcontext : cold avg=  150.204 nsec, warm avg=   38.147 nsec
unw_init_local : cold avg=  288.486 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1827.876 min= 1341.956 avg= 1396.537 nsec
global cache    : unw_step : 1st=  342.869 min=  170.299 avg=  179.509 nsec
per-thread cache: unw_step : 1st=  295.185 min=  170.299 avg=  175.034 nsec

unw_getcontext : cold avg=  159.740 nsec, warm avg=   50.068 nsec
unw_init_local : cold avg=  290.871 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1914.161 min= 1360.121 avg= 1415.378 nsec
global cache    : unw_step : 1st=  283.832 min=  140.780 avg=  145.469 nsec
per-thread cache: unw_step : 1st=  286.102 min=  131.698 avg=  141.995 nsec

unw_getcontext : cold avg=  150.204 nsec, warm avg=   47.684 nsec
unw_init_local : cold avg=  271.797 nsec, warm avg=   47.684 nsec
no cache        : unw_step : 1st= 1839.229 min= 1341.956 avg= 1428.347 nsec
global cache    : unw_step : 1st=  295.185 min=  131.698 avg=  140.117 nsec
per-thread cache: unw_step : 1st=  286.102 min=  131.698 avg=  164.806 nsec


--- fix #2 ---

unw_getcontext : cold avg=  159.740 nsec, warm avg=   38.147 nsec
unw_init_local : cold avg=  278.950 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 2132.143 min= 1341.956 avg= 1433.247 nsec
global cache    : unw_step : 1st=  381.470 min=  161.216 avg=  167.335 nsec
per-thread cache: unw_step : 1st=  351.951 min=  161.216 avg=  163.721 nsec

unw_getcontext : cold avg=  138.283 nsec, warm avg=   40.531 nsec
unw_init_local : cold avg=  259.876 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1868.748 min= 1360.121 avg= 1410.736 nsec
global cache    : unw_step : 1st=  351.951 min=  161.216 avg=  167.386 nsec
per-thread cache: unw_step : 1st=  304.268 min=  161.216 avg=  164.086 nsec

unw_getcontext : cold avg=  140.667 nsec, warm avg=   50.068 nsec
unw_init_local : cold avg=  269.413 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1836.958 min= 1341.956 avg= 1377.523 nsec
global cache    : unw_step : 1st=  390.552 min=  161.216 avg=  174.246 nsec
per-thread cache: unw_step : 1st=  447.319 min=  161.216 avg=  185.533 nsec

unw_getcontext : cold avg=  150.204 nsec, warm avg=   50.068 nsec
unw_init_local : cold avg=  290.871 nsec, warm avg=   50.068 nsec
no cache        : unw_step : 1st= 1923.243 min= 1360.121 avg= 1430.753 nsec
global cache    : unw_step : 1st=  324.703 min=  161.216 avg=  167.917 nsec
per-thread cache: unw_step : 1st=  283.832 min=  161.216 avg=  163.593 nsec


-- 
Paul Pluzhnikov