Hadoop重點難點：Shuffle過程中的環(huán)形緩沖區(qū)
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這篇文章來自一個讀者在面試過程中的一個問題，Hadoop在shuffle過程中使用了一個數(shù)據(jù)結構-環(huán)形緩沖區(qū)。
環(huán)形隊列是在實際編程極為有用的數(shù)據(jù)結構，它是一個首尾相連的FIFO的數(shù)據(jù)結構，采用數(shù)組的線性空間,數(shù)據(jù)組織簡單。能很快知道隊列是否滿為空。能以很快速度的來存取數(shù)據(jù)。 因為有簡單高效的原因，甚至在硬件都實現(xiàn)了環(huán)形隊列。
環(huán)形隊列廣泛用于網(wǎng)絡數(shù)據(jù)收發(fā)，和不同程序間數(shù)據(jù)交換（比如內核與應用程序大量交換數(shù)據(jù)，從硬件接收大量數(shù)據(jù)）均使用了環(huán)形隊列。
環(huán)形緩沖區(qū)數(shù)據(jù)結構
Map過程中環(huán)形緩沖區(qū)是指數(shù)據(jù)被map處理之后會先放入內存，內存中的這片區(qū)域就是環(huán)形緩沖區(qū)。
環(huán)形緩沖區(qū)是在MapTask.MapOutputBuffer中定義的，相關的屬性如下：
// k/v accounting |
環(huán)形緩沖區(qū)其實是一個數(shù)組，數(shù)組中存放著key、value的序列化數(shù)據(jù)和key、value的元數(shù)據(jù)信息，key/value的元數(shù)據(jù)存儲的格式是int類型，每個key/value對應一個元數(shù)據(jù)，元數(shù)據(jù)由4個int組成，第一個int存放value的起始位置，第二個存放key的起始位置，第三個存放partition，最后一個存放value的長度。
key/value序列化的數(shù)據(jù)和元數(shù)據(jù)在環(huán)形緩沖區(qū)中的存儲是由equator分隔的，key/value按照索引遞增的方向存儲，meta則按照索引遞減的方向存儲，將其數(shù)組抽象為一個環(huán)形結構之后，以equator為界，key/value順時針存儲，meta逆時針存儲。
初始化
環(huán)形緩沖區(qū)的結構在MapOutputBuffer.init中創(chuàng)建。
public void init(MapOutputCollector.Context context |
init是對環(huán)形緩沖區(qū)進行初始化構造，由mapreduce.task.io.sort.mb決定map中環(huán)形緩沖區(qū)的大小sortmb，默認是100M。
此緩沖區(qū)也用于存放meta，一個meta占用METASIZE(16byte)，則其中用于存放數(shù)據(jù)的大小是maxMemUsage -= sortmb << 20 % METASIZE(由此可知最好設置sortmb轉換為byte之后是16的整數(shù)倍)，然后用maxMemUsage初始化kvbuffer字節(jié)數(shù)組和kvmeta整形數(shù)組，最后設置數(shù)組的一些標識信息。利用setEquator(0)設置kvbuffer和kvmeta的分界線，初始化的時候以0為分界線，kvindex為aligned - METASIZE + kvbuffer.length，其位置在環(huán)形數(shù)組中相當于按照逆時針方向減去METASIZE，由kvindex設置kvstart = kvend = kvindex，由equator設置bufstart = bufend = bufindex = equator，還得設置bufvoid = kvbuffer.length，bufvoid用于標識用于存放數(shù)據(jù)的最大位置。
為了提高效率，當buffer占用達到閾值之后，會進行spill，這個閾值是由bufferRemaining進行檢查的，bufferRemaining由softLimit = (int)(kvbuffer.length * spillper); bufferRemaining = softLimit;進行初始化賦值，這里需要注意的是softLimit并不是sortmb*spillper，而是kvbuffer.length * spillper，當sortmb << 20是16的整數(shù)倍時，才可以認為softLimit是sortmb*spillper。
下面是setEquator的代碼
// setEquator(0)的代碼如下
private void setEquator(int pos) {
equator = pos;
// set index prior to first entry, aligned at meta boundary
// 第一個 entry的末尾位置，即元數(shù)據(jù)和kv數(shù)據(jù)的分界線 單位是byte
final int aligned = pos - (pos % METASIZE);
// Cast one of the operands to long to avoid integer overflow
// 元數(shù)據(jù)中存放數(shù)據(jù)的起始位置
kvindex = (int)
(((long)aligned - METASIZE + kvbuffer.length) % kvbuffer.length) / 4;
LOG.info("(EQUATOR) " + pos + " kvi " + kvindex +
"(" + (kvindex * 4) + ")");
}
buffer初始化之后的抽象數(shù)據(jù)結構如下圖所示：
環(huán)形緩沖區(qū)數(shù)據(jù)結構圖
寫入buffer
Map通過NewOutputCollector.write方法調用collector.collect向buffer中寫入數(shù)據(jù)，數(shù)據(jù)寫入之前已在NewOutputCollector.write中對要寫入的數(shù)據(jù)進行逐條分區(qū)，下面看下collect
// MapOutputBuffer.collect
public synchronized void collect(K key, V value, final int partition
) throws IOException {
...
// 新數(shù)據(jù)collect時，先將剩余的空間減去元數(shù)據(jù)的長度，之后進行判斷
bufferRemaining -= METASIZE;
if (bufferRemaining <= 0) {
// start spill if the thread is not running and the soft limit has been
// reached
spillLock.lock();
try {
do {
// 首次spill時，spillInProgress是false
if (!spillInProgress) {
// 得到kvindex的byte位置
final int kvbidx = 4 * kvindex;
// 得到kvend的byte位置
final int kvbend = 4 * kvend;
// serialized, unspilled bytes always lie between kvindex and
// bufindex, crossing the equator. Note that any void space
// created by a reset must be included in "used" bytes
final int bUsed = distanceTo(kvbidx, bufindex);
final boolean bufsoftlimit = bUsed >= softLimit;
if ((kvbend + METASIZE) % kvbuffer.length !=
equator - (equator % METASIZE)) {
// spill finished, reclaim space
resetSpill();
bufferRemaining = Math.min(
distanceTo(bufindex, kvbidx) - 2 * METASIZE,
softLimit - bUsed) - METASIZE;
continue;
} else if (bufsoftlimit && kvindex != kvend) {
// spill records, if any collected; check latter, as it may
// be possible for metadata alignment to hit spill pcnt
startSpill();
final int avgRec = (int)
(mapOutputByteCounter.getCounter() /
mapOutputRecordCounter.getCounter());
// leave at least half the split buffer for serialization data
// ensure that kvindex >= bufindex
final int distkvi = distanceTo(bufindex, kvbidx);
final int newPos = (bufindex +
Math.max(2 * METASIZE - 1,
Math.min(distkvi / 2,
distkvi / (METASIZE + avgRec) * METASIZE)))
% kvbuffer.length;
setEquator(newPos);
bufmark = bufindex = newPos;
final int serBound = 4 * kvend;
// bytes remaining before the lock must be held and limits
// checked is the minimum of three arcs: the metadata space, the
// serialization space, and the soft limit
bufferRemaining = Math.min(
// metadata max
distanceTo(bufend, newPos),
Math.min(
// serialization max
distanceTo(newPos, serBound),
// soft limit
softLimit)) - 2 * METASIZE;
}
}
} while (false);
} finally {
spillLock.unlock();
}
}
// 將key value 及元數(shù)據(jù)信息寫入緩沖區(qū)
try {
// serialize key bytes into buffer
int keystart = bufindex;
// 將key序列化寫入kvbuffer中，并移動bufindex
keySerializer.serialize(key);
// key所占空間被bufvoid分隔，則移動key，
// 將其值放在連續(xù)的空間中便于sort時key的對比
if (bufindex < keystart) {
// wrapped the key; must make contiguous
bb.shiftBufferedKey();
keystart = 0;
}
// serialize value bytes into buffer
final int valstart = bufindex;
valSerializer.serialize(value);
// It's possible for records to have zero length, i.e. the serializer
// will perform no writes. To ensure that the boundary conditions are
// checked and that the kvindex invariant is maintained, perform a
// zero-length write into the buffer. The logic monitoring this could be
// moved into collect, but this is cleaner and inexpensive. For now, it
// is acceptable.
bb.write(b0, 0, 0);
// the record must be marked after the preceding write, as the metadata
// for this record are not yet written
int valend = bb.markRecord();
mapOutputRecordCounter.increment(1);
mapOutputByteCounter.increment(
distanceTo(keystart, valend, bufvoid));
// write accounting info
kvmeta.put(kvindex + PARTITION, partition);
kvmeta.put(kvindex + KEYSTART, keystart);
kvmeta.put(kvindex + VALSTART, valstart);
kvmeta.put(kvindex + VALLEN, distanceTo(valstart, valend));
// advance kvindex
kvindex = (kvindex - NMETA + kvmeta.capacity()) % kvmeta.capacity();
} catch (MapBufferTooSmallException e) {
LOG.info("Record too large for in-memory buffer: " + e.getMessage());
spillSingleRecord(key, value, partition);
mapOutputRecordCounter.increment(1);
return;
}
}
每次寫入數(shù)據(jù)時，執(zhí)行bufferRemaining -= METASIZE之后，檢查bufferRemaining，
如果大于0，直接將key/value序列化對和對應的meta寫入buffer中，key/value是序列化之后寫入的，key/value經(jīng)過一些列的方法調用Serializer.serialize(key/value) -> WritableSerializer.serialize(key/value) -> BytesWritable.write(dataOut) -> DataOutputStream.write(bytes, 0, size) -> MapOutputBuffer.Buffer.write(b, off, len)，最后由MapOutputBuffer.Buffer.write(b, off, len)將數(shù)據(jù)寫入kvbuffer中，write方法如下：
public void write(byte b[], int off, int len)
throws IOException {
// must always verify the invariant that at least METASIZE bytes are
// available beyond kvindex, even when len == 0
bufferRemaining -= len;
if (bufferRemaining <= 0) {
// writing these bytes could exhaust available buffer space or fill
// the buffer to soft limit. check if spill or blocking are necessary
boolean blockwrite = false;
spillLock.lock();
try {
do {
checkSpillException();
final int kvbidx = 4 * kvindex;
final int kvbend = 4 * kvend;
// ser distance to key index
final int distkvi = distanceTo(bufindex, kvbidx);
// ser distance to spill end index
final int distkve = distanceTo(bufindex, kvbend);
// if kvindex is closer than kvend, then a spill is neither in
// progress nor complete and reset since the lock was held. The
// write should block only if there is insufficient space to
// complete the current write, write the metadata for this record,
// and write the metadata for the next record. If kvend is closer,
// then the write should block if there is too little space for
// either the metadata or the current write. Note that collect
// ensures its metadata requirement with a zero-length write
blockwrite = distkvi <= distkve
? distkvi <= len + 2 * METASIZE
: distkve <= len || distanceTo(bufend, kvbidx) < 2 * METASIZE;
if (!spillInProgress) {
if (blockwrite) {
if ((kvbend + METASIZE) % kvbuffer.length !=
equator - (equator % METASIZE)) {
// spill finished, reclaim space
// need to use meta exclusively; zero-len rec & 100% spill
// pcnt would fail
resetSpill(); // resetSpill doesn't move bufindex, kvindex
bufferRemaining = Math.min(
distkvi - 2 * METASIZE,
softLimit - distanceTo(kvbidx, bufindex)) - len;
continue;
}
// we have records we can spill; only spill if blocked
if (kvindex != kvend) {
startSpill();
// Blocked on this write, waiting for the spill just
// initiated to finish. Instead of repositioning the marker
// and copying the partial record, we set the record start
// to be the new equator
setEquator(bufmark);
} else {
// We have no buffered records, and this record is too large
// to write into kvbuffer. We must spill it directly from
// collect
final int size = distanceTo(bufstart, bufindex) + len;
setEquator(0);
bufstart = bufend = bufindex = equator;
kvstart = kvend = kvindex;
bufvoid = kvbuffer.length;
throw new MapBufferTooSmallException(size + " bytes");
}
}
}
if (blockwrite) {
// wait for spill
try {
while (spillInProgress) {
reporter.progress();
spillDone.await();
}
} catch (InterruptedException e) {
throw new IOException(
"Buffer interrupted while waiting for the writer", e);
}
}
} while (blockwrite);
} finally {
spillLock.unlock();
}
}
// here, we know that we have sufficient space to write
if (bufindex + len > bufvoid) {
final int gaplen = bufvoid - bufindex;
System.arraycopy(b, off, kvbuffer, bufindex, gaplen);
len -= gaplen;
off += gaplen;
bufindex = 0;
}
System.arraycopy(b, off, kvbuffer, bufindex, len);
bufindex += len;
}
write方法將key/value寫入kvbuffer中，如果bufindex+len超過了bufvoid，則將寫入的內容分開存儲，將一部分寫入bufindex和bufvoid之間，然后重置bufindex，將剩余的部分寫入，這里不區(qū)分key和value，寫入key之后會在collect中判斷bufindex < keystart，當bufindex小時，則key被分開存儲，執(zhí)行bb.shiftBufferedKey()，value則直接寫入，不用判斷是否被分開存儲，key不能分開存儲是因為要對key進行排序。
這里需要注意的是要寫入的數(shù)據(jù)太長，并且kvinde==kvend，則拋出MapBufferTooSmallException異常，在collect中捕獲，將此數(shù)據(jù)直接spill到磁盤spillSingleRecord，也就是當單條記錄過長時，不寫buffer，直接寫入磁盤。
下面看下bb.shiftBufferedKey()代碼
// BlockingBuffer.shiftBufferedKey
protected void shiftBufferedKey() throws IOException {
// spillLock unnecessary; both kvend and kvindex are current
int headbytelen = bufvoid - bufmark;
bufvoid = bufmark;
final int kvbidx = 4 * kvindex;
final int kvbend = 4 * kvend;
final int avail =
Math.min(distanceTo(0, kvbidx), distanceTo(0, kvbend));
if (bufindex + headbytelen < avail) {
System.arraycopy(kvbuffer, 0, kvbuffer, headbytelen, bufindex);
System.arraycopy(kvbuffer, bufvoid, kvbuffer, 0, headbytelen);
bufindex += headbytelen;
bufferRemaining -= kvbuffer.length - bufvoid;
} else {
byte[] keytmp = new byte[bufindex];
System.arraycopy(kvbuffer, 0, keytmp, 0, bufindex);
bufindex = 0;
out.write(kvbuffer, bufmark, headbytelen);
out.write(keytmp);
}
}
shiftBufferedKey時，判斷首部是否有足夠的空間存放key，有沒有足夠的空間，則先將首部的部分key寫入keytmp中，然后分兩次寫入，再次調用Buffer.write，如果有足夠的空間，分兩次copy，先將首部的部分key復制到headbytelen的位置，然后將末尾的部分key復制到首部，移動bufindex，重置bufferRemaining的值。
key/value寫入之后，繼續(xù)寫入元數(shù)據(jù)信息并重置kvindex的值。
spill
一次寫入buffer結束，當寫入數(shù)據(jù)比較多，bufferRemaining小于等于0時，準備進行spill，首次spill，spillInProgress為false，此時查看bUsed = distanceTo(kvbidx, bufindex)，此時bUsed >= softLimit 并且 (kvbend + METASIZE) % kvbuffer.length == equator - (equator % METASIZE)，則進行spill，調用startSpill
private void startSpill() {
// 元數(shù)據(jù)的邊界賦值
kvend = (kvindex + NMETA) % kvmeta.capacity();
// key/value的邊界賦值
bufend = bufmark;
// 設置spill運行標識
spillInProgress = true;
...
// 利用重入鎖，對spill線程進行喚醒
spillReady.signal();
}
startSpill喚醒spill線程之后，進程spill操作，但此時map向buffer的寫入操作并沒有阻塞，需要重新邊界equator和bufferRemaining的值，先來看下equator和bufferRemaining值的設定：
// 根據(jù)已經(jīng)寫入的kv得出每個record的平均長度 |
因為equator是kvbuffer和kvmeta的分界線，為了更多的空間存儲kv，則最多拿出distkvi的一半來存儲meta，并且利用avgRec估算distkvi能存放多少個record和meta對，根據(jù)record和meta對的個數(shù)估算meta所占空間的大小，從distkvi/2和meta所占空間的大小中取最小值，又因為distkvi中最少得存放一個meta，所占空間為METASIZE，在選取kvindex時需要求aligned，aligned最多為METASIZE-1，總和上述因素，最終選取equator為(bufindex + Math.max(2 * METASIZE - 1, Math.min(distkvi / 2, distkvi / (METASIZE + avgRec) * METASIZE)))。equator選取之后，設置bufmark = bufindex = newPos和kvindex，但此時并不設置bufstart、bufend和kvstart、kvend，因為這幾個值要用來表示spill數(shù)據(jù)的邊界。
spill之后，可用的空間減少了，則控制spill的bufferRemaining也應該重新設置，bufferRemaining取三個值的最小值減去2*METASIZE，三個值分別是meta可用占用的空間distanceTo(bufend, newPos),kv可用空間distanceTo(newPos, serBound)和softLimit。這里為什么要減去2*METASIZE，一個是spill之前kvend到kvindex的距離，另一個是當時的kvindex空間？？？？此時，已有一個record要寫入buffer，需要從bufferRemaining中減去當前record的元數(shù)據(jù)占用的空間，即減去METASIZE，另一個METASIZE是在計算equator時，沒有包括kvindex到kvend(spill之前)的這段METASIZE，所以要減去這個METASIZE。
接下來解析下SpillThread線程，查看其run方法：
public void run() { |
run中主要是sortAndSpill，
private void sortAndSpill() throws IOException, ClassNotFoundException,
InterruptedException {
//approximate the length of the output file to be the length of the
//buffer + header lengths for the partitions
final long size = distanceTo(bufstart, bufend, bufvoid) +
partitions * APPROX_HEADER_LENGTH;
FSDataOutputStream out = null;
try {
// create spill file
// 用來存儲index文件
final SpillRecord spillRec = new SpillRecord(partitions);
// 創(chuàng)建寫入磁盤的spill文件
final Path filename =
mapOutputFile.getSpillFileForWrite(numSpills, size);
// 打開文件流
out = rfs.create(filename);
// kvend/4 是截止到當前位置能存放多少個元數(shù)據(jù)實體
final int mstart = kvend / NMETA;
// kvstart 處能存放多少個元數(shù)據(jù)實體
// 元數(shù)據(jù)則在mstart和mend之間，(mstart - mend)則是元數(shù)據(jù)的個數(shù)
final int mend = 1 + // kvend is a valid record
(kvstart >= kvend
? kvstart
: kvmeta.capacity() + kvstart) / NMETA;
// 排序 只對元數(shù)據(jù)進行排序,只調整元數(shù)據(jù)在kvmeta中的順序
// 排序規(guī)則是MapOutputBuffer.compare，
// 先對partition進行排序其次對key值排序
sorter.sort(MapOutputBuffer.this, mstart, mend, reporter);
int spindex = mstart;
// 創(chuàng)建rec，用于存放該分區(qū)在數(shù)據(jù)文件中的信息
final IndexRecord rec = new IndexRecord();
final InMemValBytes value = new InMemValBytes();
for (int i = 0; i < partitions; ++i) {
// 臨時文件是IFile格式的
IFile.Writer<K, V> writer = null;
try {
long segmentStart = out.getPos();
FSDataOutputStream partitionOut = CryptoUtils.wrapIfNecessary(job, out);
writer = new Writer<K, V>(job, partitionOut, keyClass, valClass, codec,
spilledRecordsCounter);
// 往磁盤寫數(shù)據(jù)時先判斷是否有combiner
if (combinerRunner == null) {
// spill directly
DataInputBuffer key = new DataInputBuffer();
// 寫入相同partition的數(shù)據(jù)
while (spindex < mend &&
kvmeta.get(offsetFor(spindex % maxRec) + PARTITION) == i) {
final int kvoff = offsetFor(spindex % maxRec);
int keystart = kvmeta.get(kvoff + KEYSTART);
int valstart = kvmeta.get(kvoff + VALSTART);
key.reset(kvbuffer, keystart, valstart - keystart);
getVBytesForOffset(kvoff, value);
writer.append(key, value);
++spindex;
}
} else {
int spstart = spindex;
while (spindex < mend &&
kvmeta.get(offsetFor(spindex % maxRec)
+ PARTITION) == i) {
++spindex;
}
// Note: we would like to avoid the combiner if we've fewer
// than some threshold of records for a partition
if (spstart != spindex) {
combineCollector.setWriter(writer);
RawKeyValueIterator kvIter =
new MRResultIterator(spstart, spindex);
combinerRunner.combine(kvIter, combineCollector);
}
}
// close the writer
writer.close();
// record offsets
// 記錄當前partition i的信息寫入索文件rec中
rec.startOffset = segmentStart;
rec.rawLength = writer.getRawLength() + CryptoUtils.cryptoPadding(job);
rec.partLength = writer.getCompressedLength() + CryptoUtils.cryptoPadding(job);
// spillRec中存放了spill中partition的信息，便于后續(xù)堆排序時，取出partition相關的數(shù)據(jù)進行排序
spillRec.putIndex(rec, i);
writer = null;
} finally {
if (null != writer) writer.close();
}
}
// 判斷內存中的index文件是否超出閾值，超出則將index文件寫入磁盤
// 當超出閾值時只是把當前index和之后的index寫入磁盤
if (totalIndexCacheMemory >= indexCacheMemoryLimit) {
// create spill index file
// 創(chuàng)建index文件
Path indexFilename =
mapOutputFile.getSpillIndexFileForWrite(numSpills, partitions
* MAP_OUTPUT_INDEX_RECORD_LENGTH);
spillRec.writeToFile(indexFilename, job);
} else {
indexCacheList.add(spillRec);
totalIndexCacheMemory +=
spillRec.size() * MAP_OUTPUT_INDEX_RECORD_LENGTH;
}
LOG.info("Finished spill " + numSpills);
++numSpills;
} finally {
if (out != null) out.close();
}
}
sortAndSpill中，有mstart和mend得到一共有多少條record需要spill到磁盤，調用sorter.sort對meta進行排序，先對partition進行排序，然后按key排序，排序的結果只調整meta的順序。
排序之后，判斷是否有combiner，沒有則直接將record寫入磁盤，寫入時是一個partition一個IndexRecord，如果有combiner，則將該partition的record寫入kvIter，然后調用combinerRunner.combine執(zhí)行combiner。
寫入磁盤之后，將spillx.out對應的spillRec放入內存indexCacheList.add(spillRec)，如果所占內存totalIndexCacheMemory超過了indexCacheMemoryLimit，則創(chuàng)建index文件，將此次及以后的spillRec寫入index文件存入磁盤。
最后spill次數(shù)遞增。sortAndSpill結束之后，回到run方法中，執(zhí)行finally中的代碼，對kvstart和bufstart賦值，kvstart = kvend，bufstart = bufend，設置spillInProgress的狀態(tài)為false。
在spill的同時，map往buffer的寫操作并沒有停止，依然在調用collect，再次回到collect方法中，
// MapOutputBuffer.collect
public synchronized void collect(K key, V value, final int partition
) throws IOException {
...
// 新數(shù)據(jù)collect時，先將剩余的空間減去元數(shù)據(jù)的長度，之后進行判斷
bufferRemaining -= METASIZE;
if (bufferRemaining <= 0) {
// start spill if the thread is not running and the soft limit has been
// reached
spillLock.lock();
try {
do {
// 首次spill時，spillInProgress是false
if (!spillInProgress) {
// 得到kvindex的byte位置
final int kvbidx = 4 * kvindex;
// 得到kvend的byte位置
final int kvbend = 4 * kvend;
// serialized, unspilled bytes always lie between kvindex and
// bufindex, crossing the equator. Note that any void space
// created by a reset must be included in "used" bytes
final int bUsed = distanceTo(kvbidx, bufindex);
final boolean bufsoftlimit = bUsed >= softLimit;
if ((kvbend + METASIZE) % kvbuffer.length !=
equator - (equator % METASIZE)) {
// spill finished, reclaim space
resetSpill();
bufferRemaining = Math.min(
distanceTo(bufindex, kvbidx) - 2 * METASIZE,
softLimit - bUsed) - METASIZE;
continue;
} else if (bufsoftlimit && kvindex != kvend) {
...
}
}
} while (false);
} finally {
spillLock.unlock();
}
}
...
}
有新的record需要寫入buffer時，判斷bufferRemaining -= METASIZE，此時的bufferRemaining是在開始spill時被重置過的(此時的bufferRemaining應該比初始的softLimit要小)，當bufferRemaining小于等最后一個METASIZE是當前record進入collect之后bufferRemaining減去的那個METASIZE。
于0時，進入if，此時spillInProgress的狀態(tài)為false，進入if (!spillInProgress)，startSpill時對kvend和bufend進行了重置，則此時(kvbend + METASIZE) % kvbuffer.length != equator - (equator % METASIZE)，調用resetSpill()，將kvstart、kvend和bufstart、bufend設置為上次startSpill時的位置。此時buffer已將一部分內容寫入磁盤，有大量空余的空間，則對bufferRemaining進行重置，此次不spill。
bufferRemaining取值為Math.min(distanceTo(bufindex, kvbidx) - 2 * METASIZE, softLimit - bUsed) - METASIZE
private void resetSpill() { |
當bufferRemaining再次小于等于0時，進行spill，這以后就都是套路了。環(huán)形緩沖區(qū)分析到此結束。
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