From 825dc783e6d5da338ac21b113927ac0f2ecafe4b Mon Sep 17 00:00:00 2001
From: Sebastian Sumpf <sebastian.sumpf@genode-labs.com>
Date: Tue, 18 Sep 2012 18:49:04 +0200
Subject: [PATCH] libc: Change malloc's allocator

Use slab allocators for small object sizes, do it the usual way otherwise.

This patch is related to #363. Using this optimization may be a viable
alternative to switching to the FreeBSD's malloc implementation.
---
 libports/src/lib/libc/malloc.cc | 158 ++++++++++++++++++++++++++++----
 1 file changed, 140 insertions(+), 18 deletions(-)

diff --git a/libports/src/lib/libc/malloc.cc b/libports/src/lib/libc/malloc.cc
index 620cb44f49..f2a6308db3 100644
--- a/libports/src/lib/libc/malloc.cc
+++ b/libports/src/lib/libc/malloc.cc
@@ -1,6 +1,7 @@
 /*
  * \brief  Simplistic malloc and free implementation
  * \author Norman Feske
+ * \author Sebastian Sumpf
  * \date   2006-07-21
  */
 
@@ -14,6 +15,7 @@
 /* Genode includes */
 #include <base/env.h>
 #include <base/printf.h>
+#include <base/slab.h>
 #include <util/string.h>
 #include <util/misc_math.h>
 
@@ -22,25 +24,146 @@
 
 typedef unsigned long Block_header;
 
+namespace Genode {
+
+	class Slab_alloc : public Slab
+	{
+			size_t _calculate_block_size(size_t object_size)
+			{
+				size_t block_size = 8 * (object_size + sizeof(Slab_entry)) + sizeof(Slab_block);
+				return align_addr(block_size, 12);
+			}
+
+		public:
+
+			Slab_alloc(size_t object_size, Allocator *backing_store)
+			: Slab(object_size, _calculate_block_size(object_size), 0, backing_store)
+			{ }
+
+			inline void *alloc()
+			{
+				void *result;
+				return (Slab::alloc(slab_size(), &result) ? result : 0);
+			}
+	};
+}
+
+
+/**
+ * Allocator that uses slabs for small objects sizes
+ */
+class Malloc : public Genode::Allocator
+{
+	private:
+
+		enum {
+			SLAB_START = 2,  /* 4 Byte (log2) */
+			SLAB_STOP  = 11, /* 2048 Byte (log2) */
+			NUM_SLABS = (SLAB_STOP - SLAB_START) + 1
+		};
+
+		Genode::Allocator  *_backing_store;        /* back-end allocator */
+		Genode::Slab_alloc *_allocator[NUM_SLABS]; /* slab allocators */
+		Genode::Lock        _lock;
+
+		unsigned long _slab_log2(unsigned long size)
+		{
+			unsigned msb = Genode::log2(size);
+			/* size is greater than msb */
+			if (size > (1U << msb))
+				msb++;
+
+			/* use smallest slab */
+			if (size < (1U << SLAB_START))
+				msb = SLAB_START;
+
+			return msb;
+		}
+
+	public:
+
+		Malloc(Genode::Allocator *backing_store) : _backing_store(backing_store)
+		{
+			for (unsigned i = SLAB_START; i <= SLAB_STOP; i++) {
+				_allocator[i - SLAB_START] = new (backing_store)
+				                                 Genode::Slab_alloc(1U << i, backing_store);
+			}
+		}
+
+		/**
+		 * Allocator interface
+		 */
+
+		bool alloc(size_t size, void **out_addr)
+		{
+			Genode::Lock::Guard lock_guard(_lock);
+
+			/* enforce size to be a multiple of 4 bytes */
+			size = (size + 3) & ~3;
+
+			/*
+			 * We store the size of the allocation at the very
+			 * beginning of the allocated block and return
+			 * the subsequent address. This way, we can retrieve
+			 * the size information when freeing the block.
+			 */
+			unsigned long real_size = size + sizeof(Block_header);
+			unsigned long msb = _slab_log2(real_size);
+			void *addr = 0;
+
+			/* use backing store if requested memory is larger than largest slab */
+			if (msb > SLAB_STOP) {
+
+				if (!(_backing_store->alloc(real_size, &addr)))
+					return false;
+			}
+			else
+				if (!(addr = _allocator[msb - SLAB_START]->alloc()))
+					return false;
+
+			*(Block_header *)addr = real_size;
+			*out_addr = (Block_header *)addr + 1;
+			return true;
+		}
+
+		void free(void *ptr, size_t /* size */)
+		{
+			Genode::Lock::Guard lock_guard(_lock);
+
+			unsigned long *addr = ((unsigned long *)ptr) - 1;
+			unsigned long  real_size = *addr;
+
+			if (real_size > (1U << SLAB_STOP))
+				_backing_store->free(addr, real_size);
+			else {
+				unsigned long msb = _slab_log2(real_size);
+				_allocator[msb - SLAB_START]->free(addr);
+			}
+		}
+
+		size_t overhead(size_t size)
+		{
+			size += sizeof(Block_header);
+
+			if (size > (1U << SLAB_STOP))
+				return _backing_store->overhead(size);
+
+			return _allocator[_slab_log2(size) - SLAB_START]->overhead(size);
+		}
+};
+
+
+static Genode::Allocator *allocator()
+{
+	static Malloc _m(Genode::env()->heap());
+	return &_m;
+}
+
 
 extern "C" void *malloc(unsigned size)
 {
-	/* enforce size to be a multiple of 4 bytes */
-	size = (size + 3) & ~3;
-
-	/*
-	 * We store the size of the allocation at the very
-	 * beginning of the allocated block and return
-	 * the subsequent address. This way, we can retrieve
-	 * the size information when freeing the block.
-	 */
-	unsigned long real_size = size + sizeof(Block_header);
-	void *addr = 0;
-	if (!Genode::env()->heap()->alloc(real_size, &addr))
-		return 0;
-
-	*(Block_header *)addr = real_size;
-	return (Block_header *)addr + 1;
+	void *addr;
+	return allocator()->alloc(size, &addr) ? addr : 0;
 }
 
 
@@ -56,8 +179,7 @@ extern "C" void free(void *ptr)
 {
 	if (!ptr) return;
 
-	unsigned long *addr = ((unsigned long *)ptr) - 1;
-	Genode::env()->heap()->free(addr, *addr);
+	allocator()->free(ptr, 0);
 }