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[Engine] Defer the collection of decode inputs in prefill #2668

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Jul 18, 2024
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[Engine] Defer the collection of decode inputs in prefill #2668

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23 changes: 13 additions & 10 deletions cpp/serve/engine_actions/batch_prefill_base.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -62,15 +62,12 @@ BatchPrefillBaseActionObj::GetRequestStateEntriesToPrefill(EngineState estate) {
// Then we make a reduction to return the maximum common inputs.
for (int i = 0; i < static_cast<int>(models_.size()); ++i) {
std::vector<PrefillInput> prefill_inputs;
// - Try to prefill pending requests, in addition to reserved decode requests.
// - Try to prefill pending requests.
int total_input_length = 0;
int total_required_pages = num_decode_inputs;
// Reserve decode requests first.
for (const RequestStateEntry& rsentry : *running_rsentries) {
prefill_inputs.push_back(
{rsentry, rsentry->mstates[i]->num_tokens_for_next_decode, 0, /*is_decode=*/true});
total_input_length += rsentry->mstates[i]->num_tokens_for_next_decode;
}
int total_required_pages = num_decode_inputs;
int num_available_pages;
int num_running_rsentries = num_decode_inputs;
int current_total_seq_len;
Expand Down Expand Up @@ -211,14 +208,20 @@ BatchPrefillBaseActionObj::GetRequestStateEntriesToPrefill(EngineState estate) {
std::min(num_prefill_inputs, static_cast<int>(prefill_inputs_for_all_models[i].size()));
}

// If all inputs are decode inputs, since no prefill inputs can be added, skip prefill action
if (num_prefill_inputs == num_decode_inputs) {
if (num_prefill_inputs == 0) {
return {};
}

std::vector<PrefillInput> prefill_inputs(
prefill_inputs_for_all_models[0].begin(),
prefill_inputs_for_all_models[0].begin() + num_prefill_inputs);
// Add the decode requests to the prefill inputs.
std::vector<PrefillInput> prefill_inputs;
prefill_inputs.reserve(num_decode_inputs + num_prefill_inputs);
for (const RequestStateEntry& rsentry : *running_rsentries) {
prefill_inputs.push_back(
{rsentry, rsentry->mstates[0]->num_tokens_for_next_decode, 0, /*is_decode=*/true});
}
prefill_inputs.insert(prefill_inputs.end(), prefill_inputs_for_all_models[0].begin(),
prefill_inputs_for_all_models[0].begin() + num_prefill_inputs);
num_prefill_inputs += num_decode_inputs;
{
NVTXScopedRange nvtx_scope("reduction");
for (int i = 1; i < static_cast<int>(prefill_inputs_for_all_models.size()); ++i) {
Expand Down