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https://issues.apache.org/jira/browse/LUCENE-5161?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13735851#comment-13735851
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Uwe Schindler commented on LUCENE-5161:
---------------------------------------
Hi,
I will now explain the problems of SimpleFSDirectory and NIOFSDirectory and why
the OOM oocurs:
NIOFSDir uses a FileChannel to read from disk. This is generally a good idea to
support lockless transfers (on windows unfortunaetly not). The issue here is
some limitation in the internal JVM implementation: The big issue is the
garbage collector. It is impossible for the native code to read from a file
descriptor and let the results go directly to a native byte[] (e.g. a
ByteBuffer.allocate() on heap or a byte[] in RandomAccessFile), because those
are interruptible operations and are not synchronized. It may happen that JDK
invokes the kernel read() method and give it the native pointer of the byte[]
and suddenly the garbage collector jumps in (in another thread) and moves the
byte[] to defragment the heap. As the code is in the kernel, there is nothing
that can be done to prevent this code from writing outside byte[], once it was
moved. Theoretically the JVM could lock the byte[] somehow to prevent the GC
from moving it, but that is not how it is done.
Because of this problem FileChannel (and also RandomAccessFile) allocate a
DirectBuffer if the buffer passed to write is a heap ByteBuffer (see
[http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/sun/nio/ch/IOUtil.java#211]
and
[http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/sun/nio/ch/Util.java#60]).
Those direct byte buffers are allocated with SoftReferences to it, so they get
garbage collected one memory gets low. But as you see from the code, the direct
buffer is choosen to be at least the size of the requested transfer if none of
these buffers is available a new one is allocated with the transfer size). And
this s the big problem leading to the OOM. The maximum size of direct memory
allocated outside of the JVM is limited by the heap size (I think 2 times heap).
The current chunk sizes are horrible: With 2 Gigabytes on 64 bit and mayn
megabytes on 32 bit you allocate huge direct buffer outside of the JDK heap
that consume memory and it is unlikely that they are freed again. So we should
really limit the maximum size of those chunks to reasonable values. The
chunking code is working (and is tested), so we should limit those read buffers
to a sensible value.
E.g., for windows everything greater than 64 MB is useless (see some references
for transferTo). The only think we limit by the transfer size is the number of
syscalls, but for reading 500 MB of index norms it makes no difference if you
have 2 syscalls or 2000, the harddisk is the limiting factor).
For RandomAccessFile, the same is done: it internally allocates direct memory
(in fact the current JDKs implement RandomAccessFile mostly through NIO using
ByteBuffer.wrap()).
The above also explains why making a difference between 32 bit and 64 bits is
useless. The OOM occurs not because of the bit size, but morre because the
direct memory is like the Java heap a limitation by the underlying JDK. So we
should not waste all this memory. To also note: In fact to transfer 500 MB you
need at least the 500 MB byte[] as target for the transfer (using
ByteBuffer.wrap as we do in NIOFSDir) on heap, but also 500 MB in direct
memory, so we waste 1 Gigabyte!!! This is horrible inefficient.
Also note that NIOFSDir always has to copy the direct buffer to the heap bugger
so this is an overhead. It might be a good idea to implement a second
*optimized* NIOFSDir that uses DirectBuffers and does not copy all stuff to the
heap. For the direct buffer junks we can use similar code like in
ByteBufferIndexInput.
I would default the chunk size in NIOFSDir to something around 1 to 32
megabyte, e.g. 2 Megabytes on 32 bit and 8 Megabytes on 64 bit. Definitely the
current chunk sizes are way too large and waste physical memory we could use
for something else!
Maybe [~mikemccand] can do some perf tests with NIOFSDir with radically lowered
buffer sizes. I think it will not show any difference!
> review FSDirectory chunking defaults and test the chunking
> ----------------------------------------------------------
>
> Key: LUCENE-5161
> URL: https://issues.apache.org/jira/browse/LUCENE-5161
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Robert Muir
> Attachments: LUCENE-5161.patch
>
>
> Today there is a loop in SimpleFS/NIOFS:
> {code}
> try {
> do {
> final int readLength;
> if (total + chunkSize > len) {
> readLength = len - total;
> } else {
> // LUCENE-1566 - work around JVM Bug by breaking very large
> reads into chunks
> readLength = chunkSize;
> }
> final int i = file.read(b, offset + total, readLength);
> total += i;
> } while (total < len);
> } catch (OutOfMemoryError e) {
> {code}
> I bet if you look at the clover report its untested, because its fixed at
> 100MB for 32-bit users and 2GB for 64-bit users (are these defaults even
> good?!).
> Also if you call the setter on a 64-bit machine to change the size, it just
> totally ignores it. We should remove that, the setter should always work.
> And we should set it to small values in tests so this loop is actually
> executed.
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