llama-mmap: hint THP on mmap'd weights (Linux)

Issue madvise(MADV_HUGEPAGE) on the read-only file mapping used for
model weights on Linux. With a 1 GB model this drops the potential
page count from ~262K 4KB pages to ~512 2MB pages, reducing TLB
pressure and (more importantly) reducing the number of re-faults
when pages get evicted under memory pressure.

No-op on kernels where THP is disabled or set to 'never'. On
'madvise' mode (now the most common default for desktop distros),
this is an opt-in hint and requires the caller to ask. Guarded by
defined(MADV_HUGEPAGE) so it compiles cleanly on non-Linux.

Benchmark on a Skylake-SP VM (Bonsai-8B Q1_0, -fa on -ctk q8_0 -ctv
q8_0 -t 12 -ub 128): neutral on this machine (~9.5 t/s tg128 both
before and after) because the VM is not memory-constrained. The
change is intended for systems where the weights mapping gets
evicted and re-faulted under pressure, where the fault count
reduction from huge pages matters more than TLB pressure.

Author: noreply

src/llama-mmap.cpp

@@ -451,6 +451,20 @@ struct llama_mmap::impl {
             throw std::runtime_error(format("mmap failed: %s", strerror(errno)));
         }
 
+#ifdef __linux__
+        // Hint the kernel to back this region with 2MB huge pages where possible.
+        // For a 1 GB model weights map this can drop the number of pages from ~262K
+        // 4KB pages to ~512 2MB pages, reducing TLB pressure and (critically)
+        // reducing the number of re-faults when pages get evicted under memory
+        // pressure. No-op if THP is not enabled / supported.
+        if (!numa) {
+            if (madvise(addr, file->size(), MADV_HUGEPAGE)) {
+                LLAMA_LOG_DEBUG("note: madvise(.., MADV_HUGEPAGE) not applied: %s\n",
+                        strerror(errno));
+            }
+        }
+#endif
+
         if (prefetch > 0) {
             if (posix_madvise(addr, std::min(file->size(), prefetch), POSIX_MADV_WILLNEED)) {
                 LLAMA_LOG_WARN("warning: posix_madvise(.., POSIX_MADV_WILLNEED) failed: %s\n",