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r13927@catbus: nickm | 2007-07-26 17:26:49 -0400

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Fix router_choose_by_bandwidth to no longer be biases by floating-point roundoff issues.  This runs through the list of routers yet another time, and uses an additional bitfield, but this should be okay: the function did not appear in profiles before, and shouldnt start appearing now.


svn:r10939
This commit is contained in:
Nick Mathewson
2007-07-26 21:26:57 +00:00
parent b1c873182d
commit eed888a2b7
2 changed files with 25 additions and 8 deletions
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src/or/routerlist.c
+19 -8
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@@ -1200,12 +1200,14 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, int for_exit, int statuses)
uint64_t rand_bw, tmp;
double exit_weight;
int n_unknown = 0;
bitarray_t *exit_bits;
/* First count the total bandwidth weight, and make a list
* of each value. <0 means "unknown; no routerinfo." We use the
* bits of negative values to remember whether the router was fast (-x)&1
* and whether it was an exit (-x)&2. Yes, it's a hack. */
bandwidths = tor_malloc(sizeof(int32_t)*smartlist_len(sl));
exit_bits = bitarray_init_zero(smartlist_len(sl));
/* Iterate over all the routerinfo_t or routerstatus_t, and */
for (i = 0; i < (unsigned)smartlist_len(sl); ++i) {
@@ -1230,6 +1232,8 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, int for_exit, int statuses)
is_exit = router->is_exit;
this_bw = router_get_advertised_bandwidth(router);
}
if (is_exit)
bitarray_set(exit_bits, i);
/* if they claim something huge, don't believe it */
if (this_bw > MAX_BELIEVABLE_BANDWIDTH)
this_bw = MAX_BELIEVABLE_BANDWIDTH;
@@ -1293,8 +1297,18 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, int for_exit, int statuses)
uint64_t leftover = (total_exit_bw - total_nonexit_bw / 2);
exit_weight = U64_TO_DBL(leftover) /
U64_TO_DBL(leftover + total_nonexit_bw);
#if 0
total_bw = total_nonexit_bw +
DBL_TO_U64(exit_weight * U64_TO_DBL(total_exit_bw));
#endif
total_bw = 0;
for (i=0; i < (unsigned)smartlist_len(sl); i++) {
is_exit = bitarray_is_set(exit_bits, i);
if (is_exit)
total_bw += ((uint64_t)(bandwidths[i] * exit_weight));
else
total_bw += bandwidths[i];
}
}
/*
log_debug(LD_CIRC, "Total bw = "U64_FORMAT", total exit bw = "U64_FORMAT
@@ -1310,13 +1324,7 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, int for_exit, int statuses)
/* Last, count through sl until we get to the element we picked */
tmp = 0;
for (i=0; i < (unsigned)smartlist_len(sl); i++) {
if (statuses) {
status = smartlist_get(sl, i);
is_exit = status->is_exit;
} else {
router = smartlist_get(sl, i);
is_exit = router->is_exit;
}
is_exit = bitarray_is_set(exit_bits, i);
if (is_exit)
tmp += ((uint64_t)(bandwidths[i] * exit_weight));
else
@@ -1325,12 +1333,15 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, int for_exit, int statuses)
break;
}
if (i == (unsigned)smartlist_len(sl)) {
/* This is possible due to round-off error. */
/* This was once possible due to round-off error, but shouldn't be able
* to occur any longer. */
tor_fragile_assert();
--i;
log_warn(LD_BUG, "Round-off error in computing bandwidth had an effect on "
" which router we chose. Please tell the developers.");
}
tor_free(bandwidths);
tor_free(exit_bits);
return smartlist_get(sl, i);
}