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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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6
ChangeLog
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@ -85,6 +85,12 @@ Changes in version 0.2.0.3-alpha - 2007-??-??
field. "GETINFO address-mappings" always does the right thing.
- Use CRLF line endings properly in NS events.
o Minor bugfixes (misc):
- Choose perfectly fairly among routers when choosing by bandwidth and
weighting by fraction of bandwidth provided by exits. Previously,
we would choose with only approximate fairness, and correct ourselves
if we ran off the end of the list. [Bugfix on 0.1.2.x]
Changes in version 0.1.2.15 - 2007-07-17
o Major bugfixes (compilation):

27
src/or/routerlist.c
View File

@ -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);
}