1 files changed, 122 insertions, 0 deletions
diff --git a/llama.cpp/tests/test-state-restore-fragmented.cpp b/llama.cpp/tests/test-state-restore-fragmented.cpp
new file mode 100644
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--- /dev/null
+++ b/llama.cpp/tests/test-state-restore-fragmented.cpp
@@ -0,0 +1,122 @@
+// Test for state restore with fragmented KV cache
+// This tests the fix for: https://github.com/ggml-org/llama.cpp/issues/17527
+// The issue was that state restore required contiguous KV cache slots,
+// which fails when the cache is fragmented.
+//
+// The fix changes find_slot(ubatch, true) to find_slot(ubatch, false)
+// in state_read_meta(), allowing non-contiguous slot allocation.
+
+#include "arg.h"
+#include "common.h"
+#include "llama.h"
+
+#include <vector>
+#include <cstdio>
+#include <cstring>
+
+int main(int argc, char ** argv) {
+    common_params params;
+
+    params.sampling.seed = 1234;
+    params.kv_unified = true;
+    params.n_parallel = 3;
+    params.n_ctx = 256;
+
+    if (!common_params_parse(argc, argv, params, LLAMA_EXAMPLE_COMMON)) {
+        return 1;
+    }
+
+    common_init();
+
+    // init
+    common_init_result_ptr llama_init = common_init_from_params(params);
+
+    llama_model * model = llama_init->model();
+    llama_context * ctx = llama_init->context();
+
+    if (model == nullptr || ctx == nullptr) {
+        fprintf(stderr, "%s : failed to init\n", __func__);
+        return 1;
+    }
+
+    GGML_UNUSED(model);
+
+    // tokenize prompt
+    std::vector<llama_token> tokens(70, 1);
+
+    // interleave the 3 sequences:
+    // 01201230123...
+    llama_batch batch = llama_batch_init(params.n_parallel*tokens.size(), 0, 1);
+    for (size_t i = 0; i < tokens.size(); i++) {
+        for (int s = 0; s < params.n_parallel; ++s) {
+            common_batch_add(batch, tokens[i], i, {s}, false);
+        }
+    }
+    batch.logits[batch.n_tokens - 1] = true;
+
+    if (llama_decode(ctx, batch)) {
+        fprintf(stderr, "%s : failed to decode seq 0\n", __func__);
+        return 1;
+    }
+
+    fprintf(stderr, "%s : processed prompt on seq 0, 1, 2 (%zu tokens each)\n", __func__, tokens.size());
+
+    // Save state of seq 1
+    std::vector<uint8_t> seq_state(llama_state_seq_get_size(ctx, 1));
+    const size_t ncopy = llama_state_seq_get_data(ctx, seq_state.data(), seq_state.size(), 1);
+    if (ncopy != seq_state.size()) {
+        fprintf(stderr, "%s : failed to save seq 1 state\n", __func__);
+        return 1;
+    }
+    fprintf(stderr, "%s : saved seq 1 state, %zu bytes\n", __func__, ncopy);
+
+    // clear seq 1 to create a "hole" in the KV cache (fragmentation)
+    // 0.20.20.20.2....
+    llama_memory_t mem = llama_get_memory(ctx);
+    llama_memory_seq_rm(mem, 1, -1, -1);
+    fprintf(stderr, "%s : cleared seq 1 to create fragmentation\n", __func__);
+
+    // Now the cache has holes where seq 1 was
+    // This creates fragmentation - there's no contiguous block large enough
+    // for the seq 1 state if we only look for contiguous slots
+
+    // Restore seq 1 state into seq 1 (should work with non-contiguous allocation)
+    // We use seq 1 since it's a valid sequence ID (0 to n_parallel-1)
+    // Before the fix, this would fail with "failed to find available cells in kv cache"
+    const size_t nset = llama_state_seq_set_data(ctx, seq_state.data(), seq_state.size(), 1);
+    if (nset != seq_state.size()) {
+        fprintf(stderr, "%s : FAILED to restore seq state into fragmented cache (got %zu, expected %zu)\n",
+                __func__, nset, seq_state.size());
+        fprintf(stderr, "%s : This is the bug - state restore fails with fragmented KV cache\n", __func__);
+        llama_batch_free(batch);
+        return 1;
+    }
+    fprintf(stderr, "%s : restored state into seq 1, %zu bytes\n", __func__, nset);
+
+    // Verify we can decode with the restored state
+    // Generate one token to verify the restored state is usable
+    auto sparams = llama_sampler_chain_default_params();
+    llama_sampler * smpl = llama_sampler_chain_init(sparams);
+    llama_sampler_chain_add(smpl, llama_sampler_init_dist(params.sampling.seed));
+
+    auto next_token = llama_sampler_sample(smpl, ctx, -1);
+    auto next_token_str = common_token_to_piece(ctx, next_token);
+
+    common_batch_clear(batch);
+    common_batch_add(batch, next_token, (int)tokens.size(), {1}, true);
+
+    if (llama_decode(ctx, batch)) {
+        fprintf(stderr, "%s : failed to decode with restored state\n", __func__);
+        llama_sampler_free(smpl);
+        llama_batch_free(batch);
+        return 1;
+    }
+
+    fprintf(stderr, "%s : successfully decoded with restored state, generated: '%s'\n", __func__, next_token_str.c_str());
+    fprintf(stderr, "%s : SUCCESS - state restore works with fragmented KV cache\n", __func__);
+
+    llama_sampler_free(smpl);
+    llama_batch_free(batch);
+
+    return 0;
+}