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Fix bug in roulette wheel

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Slot values were changed after the total was calculated. Fix bug
and adjust the offending code.
This commit is contained in:
Niclas Antti 2018-11-06 10:03:54 +02:00
parent c692c864e2
commit 3ccdb508de
1 changed files with 7 additions and 7 deletions
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14
server/modules/routing/readwritesplit/rwsplit_select_backends.cc
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@ -122,8 +122,7 @@ SRWBackendVector::iterator backend_cmp_response_time(SRWBackendVector& sBackends
double slot[SZ];
// fill slots with inverses of averages
double total {0};
double fastest {0};
double pre_total {0};
for (int i = 0; i < SZ; ++i)
{
SERVER_REF* server = (**sBackends[i]).backend();
@ -139,16 +138,17 @@ SRWBackendVector::iterator backend_cmp_response_time(SRWBackendVector& sBackends
slot[i] = 1 / ave;
}
slot[i] = slot[i] * slot[i] * slot[i]; // favor faster servers even more
total += slot[i];
fastest = std::max(fastest, slot[i]);
pre_total += slot[i];
}
// make the slowest server at least a good fraction of the fastest to guarantee
// make the slowest server(s) at least a good fraction of the total to guarantee
// some amount of sampling, should the slow server become faster.
constexpr int divisor = 200; // 0.5%
double total = 0;
constexpr int divisor = 197; // ~0.5%, not exact because SZ>1
for (int i = 0; i < SZ; ++i)
{
slot[i] = std::max(slot[i], fastest / divisor);
slot[i] = std::max(slot[i], pre_total / divisor);
total += slot[i];
}
// turn slots into a roulette wheel, where sum of slots is 1.0