fix algorithm of spreading vectors over shards #3374
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| @@ -82,7 +82,7 @@ void GpuIcmEncoder::encode( | |||
| size_t n, | |||
| size_t ils_iters) const { | |||
| size_t nshards = shards->size(); | |||
| size_t shard_size = (n + nshards - 1) / nshards; | |||
| size_t base_shard_size = n / nshards; | |||
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is it (n / nshards) or (n - 1)/ nshards?
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integer computing, round down already. remain = n - nshards * base_shard_size, then 0 <= remain < nshards, so each shard takes at least base_shard_size vectors, and the first remain shards takes one extra vector each.
| @@ -94,8 +94,14 @@ void GpuIcmEncoder::encode( | |||
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| // split input data | |||
| auto fn = [=](int idx, IcmEncoderImpl* encoder) { | |||
| size_t i0 = idx * shard_size; | |||
| size_t ni = std::min(shard_size, n - i0); | |||
| size_t i0 = idx * base_shard_size + std::min(size_t(idx), n % nshards); | |||
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why is it std::min(size_t(idx), n % nshards)? Btw, should be (n-1) % nshards?
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as my previous comment, you can verify it over the case n % nshards == 0, then (n -1) % nshards is meaningless
simple math:
root cause:
integer cause precision loss,
idx * shard_sizeoverflows, because(n + nshards - 1) / nshardsis roundupmy solution:
each shard takes at least
base_shard_size = n / nshards, thenremain = n % nshards, we know0 <= remain < nshards, next, assign those remain vectors to firstremainshards, i.e. firstremainshards take one more vector each.simplify above code:
i0 = idx * base_shard_size + std::min(size_t(idx), n % nshards);