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The official Couchbase JVM Core IO Library
/*
* Copyright 2012 The Netty Project
*
* The Netty Project licenses this file to you under the Apache License,
* version 2.0 (the "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at:
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations
* under the License.
*/
package com.couchbase.client.deps.io.netty.buffer;
/**
* Description of algorithm for PageRun/PoolSubpage allocation from PoolChunk
*
* Notation: The following terms are important to understand the code
* > page - a page is the smallest unit of memory chunk that can be allocated
* > chunk - a chunk is a collection of pages
* > in this code chunkSize = 2^{maxOrder} * pageSize
*
* To begin we allocate a byte array of size = chunkSize
* Whenever a ByteBuf of given size needs to be created we search for the first position
* in the byte array that has enough empty space to accommodate the requested size and
* return a (long) handle that encodes this offset information, (this memory segment is then
* marked as reserved so it is always used by exactly one ByteBuf and no more)
*
* For simplicity all sizes are normalized according to PoolArena#normalizeCapacity method
* This ensures that when we request for memory segments of size >= pageSize the normalizedCapacity
* equals the next nearest power of 2
*
* To search for the first offset in chunk that has at least requested size available we construct a
* complete balanced binary tree and store it in an array (just like heaps) - memoryMap
*
* The tree looks like this (the size of each node being mentioned in the parenthesis)
*
* depth=0 1 node (chunkSize)
* depth=1 2 nodes (chunkSize/2)
* ..
* ..
* depth=d 2^d nodes (chunkSize/2^d)
* ..
* depth=maxOrder 2^maxOrder nodes (chunkSize/2^{maxOrder} = pageSize)
*
* depth=maxOrder is the last level and the leafs consist of pages
*
* With this tree available searching in chunkArray translates like this:
* To allocate a memory segment of size chunkSize/2^k we search for the first node (from left) at height k
* which is unused
*
* Algorithm:
* ----------
* Encode the tree in memoryMap with the notation
* memoryMap[id] = x => in the subtree rooted at id, the first node that is free to be allocated
* is at depth x (counted from depth=0) i.e., at depths [depth_of_id, x), there is no node that is free
*
* As we allocate & free nodes, we update values stored in memoryMap so that the property is maintained
*
* Initialization -
* In the beginning we construct the memoryMap array by storing the depth of a node at each node
* i.e., memoryMap[id] = depth_of_id
*
* Observations:
* -------------
* 1) memoryMap[id] = depth_of_id => it is free / unallocated
* 2) memoryMap[id] > depth_of_id => at least one of its child nodes is allocated, so we cannot allocate it, but
* some of its children can still be allocated based on their availability
* 3) memoryMap[id] = maxOrder + 1 => the node is fully allocated & thus none of its children can be allocated, it
* is thus marked as unusable
*
* Algorithm: [allocateNode(d) => we want to find the first node (from left) at height h that can be allocated]
* ----------
* 1) start at root (i.e., depth = 0 or id = 1)
* 2) if memoryMap[1] > d => cannot be allocated from this chunk
* 3) if left node value <= h; we can allocate from left subtree so move to left and repeat until found
* 4) else try in right subtree
*
* Algorithm: [allocateRun(size)]
* ----------
* 1) Compute d = log_2(chunkSize/size)
* 2) Return allocateNode(d)
*
* Algorithm: [allocateSubpage(size)]
* ----------
* 1) use allocateNode(maxOrder) to find an empty (i.e., unused) leaf (i.e., page)
* 2) use this handle to construct the PoolSubpage object or if it already exists just call init(normCapacity)
* note that this PoolSubpage object is added to subpagesPool in the PoolArena when we init() it
*
* Note:
* -----
* In the implementation for improving cache coherence,
* we store 2 pieces of information (i.e, 2 byte vals) as a short value in memoryMap
*
* memoryMap[id]= (depth_of_id, x)
* where as per convention defined above
* the second value (i.e, x) indicates that the first node which is free to be allocated is at depth x (from root)
*/
final class PoolChunk {
final PoolArena arena;
final T memory;
final boolean unpooled;
private final byte[] memoryMap;
private final byte[] depthMap;
private final PoolSubpage[] subpages;
/** Used to determine if the requested capacity is equal to or greater than pageSize. */
private final int subpageOverflowMask;
private final int pageSize;
private final int pageShifts;
private final int maxOrder;
private final int chunkSize;
private final int log2ChunkSize;
private final int maxSubpageAllocs;
/** Used to mark memory as unusable */
private final byte unusable;
private int freeBytes;
PoolChunkList parent;
PoolChunk prev;
PoolChunk next;
// TODO: Test if adding padding helps under contention
//private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
PoolChunk(PoolArena arena, T memory, int pageSize, int maxOrder, int pageShifts, int chunkSize) {
unpooled = false;
this.arena = arena;
this.memory = memory;
this.pageSize = pageSize;
this.pageShifts = pageShifts;
this.maxOrder = maxOrder;
this.chunkSize = chunkSize;
unusable = (byte) (maxOrder + 1);
log2ChunkSize = log2(chunkSize);
subpageOverflowMask = ~(pageSize - 1);
freeBytes = chunkSize;
assert maxOrder < 30 : "maxOrder should be < 30, but is: " + maxOrder;
maxSubpageAllocs = 1 << maxOrder;
// Generate the memory map.
memoryMap = new byte[maxSubpageAllocs << 1];
depthMap = new byte[memoryMap.length];
int memoryMapIndex = 1;
for (int d = 0; d <= maxOrder; ++ d) { // move down the tree one level at a time
int depth = 1 << d;
for (int p = 0; p < depth; ++ p) {
// in each level traverse left to right and set value to the depth of subtree
memoryMap[memoryMapIndex] = (byte) d;
depthMap[memoryMapIndex] = (byte) d;
memoryMapIndex ++;
}
}
subpages = newSubpageArray(maxSubpageAllocs);
}
/** Creates a special chunk that is not pooled. */
PoolChunk(PoolArena arena, T memory, int size) {
unpooled = true;
this.arena = arena;
this.memory = memory;
memoryMap = null;
depthMap = null;
subpages = null;
subpageOverflowMask = 0;
pageSize = 0;
pageShifts = 0;
maxOrder = 0;
unusable = (byte) (maxOrder + 1);
chunkSize = size;
log2ChunkSize = log2(chunkSize);
maxSubpageAllocs = 0;
}
@SuppressWarnings("unchecked")
private PoolSubpage[] newSubpageArray(int size) {
return new PoolSubpage[size];
}
int usage() {
final int freeBytes = this.freeBytes;
if (freeBytes == 0) {
return 100;
}
int freePercentage = (int) (freeBytes * 100L / chunkSize);
if (freePercentage == 0) {
return 99;
}
return 100 - freePercentage;
}
long allocate(int normCapacity) {
if ((normCapacity & subpageOverflowMask) != 0) { // >= pageSize
return allocateRun(normCapacity);
} else {
return allocateSubpage(normCapacity);
}
}
/**
* Update method used by allocate
* This is triggered only when a successor is allocated and all its predecessors
* need to update their state
* The minimal depth at which subtree rooted at id has some free space
*
* @param id id
*/
private void updateParentsAlloc(int id) {
while (id > 1) {
int parentId = id >>> 1;
byte val1 = value(id);
byte val2 = value(id ^ 1);
byte val = val1 < val2 ? val1 : val2;
setValue(parentId, val);
id = parentId;
}
}
/**
* Update method used by free
* This needs to handle the special case when both children are completely free
* in which case parent be directly allocated on request of size = child-size * 2
*
* @param id id
*/
private void updateParentsFree(int id) {
int logChild = depth(id) + 1;
while (id > 1) {
int parentId = id >>> 1;
byte val1 = value(id);
byte val2 = value(id ^ 1);
logChild -= 1; // in first iteration equals log, subsequently reduce 1 from logChild as we traverse up
if (val1 == logChild && val2 == logChild) {
setValue(parentId, (byte) (logChild - 1));
} else {
byte val = val1 < val2 ? val1 : val2;
setValue(parentId, val);
}
id = parentId;
}
}
/**
* Algorithm to allocate an index in memoryMap when we query for a free node
* at depth d
*
* @param d depth
* @return index in memoryMap
*/
private int allocateNode(int d) {
int id = 1;
int initial = - (1 << d); // has last d bits = 0 and rest all = 1
byte val = value(id);
if (val > d) { // unusable
return -1;
}
while (val < d || (id & initial) == 0) { // id & initial == 1 << d for all ids at depth d, for < d it is 0
id <<= 1;
val = value(id);
if (val > d) {
id ^= 1;
val = value(id);
}
}
byte value = value(id);
assert value == d && (id & initial) == 1 << d : String.format("val = %d, id & initial = %d, d = %d",
value, id & initial, d);
setValue(id, unusable); // mark as unusable
updateParentsAlloc(id);
return id;
}
/**
* Allocate a run of pages (>=1)
*
* @param normCapacity normalized capacity
* @return index in memoryMap
*/
private long allocateRun(int normCapacity) {
int d = maxOrder - (log2(normCapacity) - pageShifts);
int id = allocateNode(d);
if (id < 0) {
return id;
}
freeBytes -= runLength(id);
return id;
}
/**
* Create/ initialize a new PoolSubpage of normCapacity
* Any PoolSubpage created/ initialized here is added to subpage pool in the PoolArena that owns this PoolChunk
*
* @param normCapacity normalized capacity
* @return index in memoryMap
*/
private long allocateSubpage(int normCapacity) {
int d = maxOrder; // subpages are only be allocated from pages i.e., leaves
int id = allocateNode(d);
if (id < 0) {
return id;
}
final PoolSubpage[] subpages = this.subpages;
final int pageSize = this.pageSize;
freeBytes -= pageSize;
int subpageIdx = subpageIdx(id);
PoolSubpage subpage = subpages[subpageIdx];
if (subpage == null) {
subpage = new PoolSubpage(this, id, runOffset(id), pageSize, normCapacity);
subpages[subpageIdx] = subpage;
} else {
subpage.init(normCapacity);
}
return subpage.allocate();
}
/**
* Free a subpage or a run of pages
* When a subpage is freed from PoolSubpage, it might be added back to subpage pool of the owning PoolArena
* If the subpage pool in PoolArena has at least one other PoolSubpage of given elemSize, we can
* completely free the owning Page so it is available for subsequent allocations
*
* @param handle handle to free
*/
void free(long handle) {
int memoryMapIdx = (int) handle;
int bitmapIdx = (int) (handle >>> Integer.SIZE);
if (bitmapIdx != 0) { // free a subpage
PoolSubpage subpage = subpages[subpageIdx(memoryMapIdx)];
assert subpage != null && subpage.doNotDestroy;
if (subpage.free(bitmapIdx & 0x3FFFFFFF)) {
return;
}
}
freeBytes += runLength(memoryMapIdx);
setValue(memoryMapIdx, depth(memoryMapIdx));
updateParentsFree(memoryMapIdx);
}
void initBuf(PooledByteBuf buf, long handle, int reqCapacity) {
int memoryMapIdx = (int) handle;
int bitmapIdx = (int) (handle >>> Integer.SIZE);
if (bitmapIdx == 0) {
byte val = value(memoryMapIdx);
assert val == unusable : String.valueOf(val);
buf.init(this, handle, runOffset(memoryMapIdx), reqCapacity, runLength(memoryMapIdx));
} else {
initBufWithSubpage(buf, handle, bitmapIdx, reqCapacity);
}
}
void initBufWithSubpage(PooledByteBuf buf, long handle, int reqCapacity) {
initBufWithSubpage(buf, handle, (int) (handle >>> Integer.SIZE), reqCapacity);
}
private void initBufWithSubpage(PooledByteBuf buf, long handle, int bitmapIdx, int reqCapacity) {
assert bitmapIdx != 0;
int memoryMapIdx = (int) handle;
PoolSubpage subpage = subpages[subpageIdx(memoryMapIdx)];
assert subpage.doNotDestroy;
assert reqCapacity <= subpage.elemSize;
buf.init(
this, handle,
runOffset(memoryMapIdx) + (bitmapIdx & 0x3FFFFFFF) * subpage.elemSize, reqCapacity, subpage.elemSize);
}
private byte value(int id) {
return memoryMap[id];
}
private void setValue(int id, byte val) {
memoryMap[id] = val;
}
private byte depth(int id) {
return depthMap[id];
}
private static int log2(int val) {
// compute the (0-based, with lsb = 0) position of highest set bit i.e, log2
return Integer.SIZE - 1 - Integer.numberOfLeadingZeros(val);
}
private int runLength(int id) {
// represents the size in #bytes supported by node 'id' in the tree
return 1 << log2ChunkSize - depth(id);
}
private int runOffset(int id) {
// represents the 0-based offset in #bytes from start of the byte-array chunk
int shift = id ^ 1 << depth(id);
return shift * runLength(id);
}
private int subpageIdx(int memoryMapIdx) {
return memoryMapIdx ^ maxSubpageAllocs; // remove highest set bit, to get offset
}
@Override
public String toString() {
return new StringBuilder()
.append("Chunk(")
.append(Integer.toHexString(System.identityHashCode(this)))
.append(": ")
.append(usage())
.append("%, ")
.append(chunkSize - freeBytes)
.append('/')
.append(chunkSize)
.append(')')
.toString();
}
}