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Do not shift context for sliding window models (#5368)

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* Do not shift context for sliding window models

* truncate prompt > 2/3 tokens

* only target gemma2
This commit is contained in:
Jeffrey Morgan 2024-06-28 19:39:31 -07:00 committed by GitHub
parent 5f034f5b63
commit 717f7229eb
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GPG key ID: B5690EEEBB952194
1 changed files with 37 additions and 9 deletions
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46
llm/ext_server/server.cpp vendored
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@ -1650,26 +1650,41 @@ struct llama_server_context
}
slot.params.n_keep = std::min(slot.n_ctx - 4, slot.params.n_keep);
char buf[256];
llama_model_meta_val_str(model, "general.architecture", buf, 256);
bool gemma2 = strcmp(buf, "gemma2") == 0;
int32_t truncate_at = slot.n_ctx;
// truncate at 2/3 of the context length for gemma2 models
// as they do not support context shifts (from the sliding window implementation).
// this way, prompts that almost fit the context length can still generate a full
// response without a sudden stop from hitting the context limit
if (gemma2) {
truncate_at = 2 * slot.n_ctx / 3;
}
// if input prompt is too big, truncate it, if group attention self-extend is disabled
if (slot.ga_n == 1 && slot.n_prompt_tokens >= slot.n_ctx)
if (slot.ga_n == 1 && slot.n_prompt_tokens >= truncate_at)
{
const int n_left = slot.n_ctx - slot.params.n_keep;
const int n_block_size = n_left / 2;
const int erased_blocks = (slot.n_prompt_tokens - slot.params.n_keep - n_block_size) / n_block_size;
const int n_shift = n_left / 2;
const int n_erase = slot.n_prompt_tokens - slot.params.n_keep - n_shift;
std::vector<llama_token> new_tokens(
prompt_tokens.begin(),
prompt_tokens.begin() + slot.params.n_keep);
new_tokens.insert(
new_tokens.end(),
prompt_tokens.begin() + slot.params.n_keep + erased_blocks * n_block_size,
prompt_tokens.begin() + slot.params.n_keep + n_erase,
prompt_tokens.end());
LOG_VERBOSE("input truncated", {
{"n_ctx", slot.n_ctx},
{"n_keep", slot.params.n_keep},
{"n_left", n_left},
{"new_tokens", tokens_to_str(ctx, new_tokens.cbegin(), new_tokens.cend())},
LOG_INFO("input truncated", {
{"n_ctx", slot.n_ctx},
{"n_keep", slot.params.n_keep},
{"n_left", n_left},
{"n_shift", n_shift},
{"n_erase", n_erase},
});
slot.truncated = true;
prompt_tokens = new_tokens;
@ -1678,6 +1693,19 @@ struct llama_server_context
GGML_ASSERT(slot.n_prompt_tokens < slot.n_ctx);
}
// Models with sliding window attention do not work with context shifts, so
// limit their prediction to the context length
if (gemma2) {
int32_t limit = slot.n_ctx - slot.n_prompt_tokens;
slot.n_predict = limit;
slot.params.n_predict = limit;
LOG_INFO("model does not support sliding window, limiting generation", {
{"n_ctx", slot.n_ctx},
{"n_prompt_tokens", slot.n_prompt_tokens},
{"n_predict", slot.n_predict}
});
}
if (!slot.params.cache_prompt)
{
llama_sampling_reset(slot.ctx_sampling);