Can SocketChannel.write return -1 ?

Hi,
In the SSLEngine documentation sample code it mentions handling a -1 returned by Channel.write() if the channel is closed.
This is the only place I've ever seen it mentioned, there's nothing in the javadoc, and I've never come across it in practice.
Can anybody confirm that this is just a mistake - or is it something my code needs to handle ?
regards
Tony Seebregts

I've just submitted this as a bug. It occurs twice in the sample: once in the I/O section and once in the shutdown section. The tests for num == -1 and the associated empty code block and comment should be deleted. The code seems to have been copied & pasted from the read() block, where a -1 return is indeed valid.
Also in the shutdown section it incorrectly has
while (myNetData().hasRemaining())which should be
while (myNetData.hasRemaining())Also surely myNetData needs to be flipped before the write and compacted afterwards, which means that the test should really be not hasRemaining() but position() > 0. hasRemaining() would only work if the buffer was already flipped.

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SocketChannel.write() throws IOException instead of returning 0

I would like your opinion.
When a send buffer is full in the OS, should a channel's write()
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same exception (IOException) thrown when truly exceptional events
happen (e.g, a connection drop)?
On Win32, SocketChannel.write() internally calls WSASend(). When
WSASend() returns WSAEWOULDBLOCK, write() throws IOException. I
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Thanks,
Juan

The java doc for write() says it should return zero. If you have a simple test case that demonstrates the behavior you describe and it is not already in the bug database, then yes. You should file the bug report.

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Greets,
I'm developping a huge application and got a latency/block problem using the write(ByteBuffer) method on a Socket from a socketchannel connection.
Running java 1.5 (diablo) on Freebsd 6 servers, 4Gb ram (2.2 allocated to jvm), with dual xeon dual-core (total 4 cores)
Here is the application schema :
- A thread accepting connexion on the socketchannel
- A thread selecting keys with data to process, enqueuing it after some basic checks on a command FIFO
- A thread getting commands from the FIFO and processing 'em, generating answers on 4 answer FIFOs
- 4 threads (1 per FIFO) to get answers and send 'em back to the socket.
The application usually runs with 4500-5000 simultaneous clients.
My problem is that the only write() method sometimes takes over 20ms to write a message, with a length smaller than 50 bytes.
As I got about 25000 answers to process each second, when some of 'em decide to be slow, the 4 threads runs slowly, and all the connected clients are suffering of that latency, for the few minutes needed to empty the FIFOs.
Every client socket get about 5 answers per second.
On about 1 hour running, there are about 3 'peaks' of slowness, that I cannot explain yet. That's why I'm in need of advices !
I monitored the application when such case happens. TOP indicates 40% cpu idle, JVM memory got >500Mb free, network runs @ about 1.2Mbps, where maximal transfer rate is >20Mbps. netstat -m told me no erros, and a large amount of free buffers available.
As the only slow process is the write() method that usually runs faster than 1ms each call, but in those case I got delays over 20ms.
freebsd tcp default sendbuffer size is 64k, receive buffer is 32k
Commands average received size is below 1k, Answers average sending size below 8k.
This application is running live, and as I cannot emulate 5000+ connections with a similar beahviour to test withour being sure that won't crash all.
What points could be responsible of such slow write() calls ? Seems it's not CPU, not RAM, not network itself...
I suppose it's the network buffers that are causing problems. But I don't really know if I have to fit 'em to a lower size, fitting my requirements, or to a larger size, to be sure there won't be full buffers blocking all ?
I need advices. Thanks for your ideas !
Bill

Hmm. So you're happy to lose data?
A few comments:
(a) SocketChannels are thread-safe. I don't think you need the synchronization at all, unless maybe multiple writing threads are possible. I would eliminate that possibility and the sync myself.
(b) If you're getting write delays of 30ms occasionally, the sync must also take 30ms at the same points if it is doing anything at all, i.e. if the possibility of multiple writing threads does exist. So maybe it doesn't?
(c) I would have a good look at this:
http://forum.java.sun.com/thread.jspa?threadID=459338
and specifically the part on how to manage a channel that presents write blocks, using OP_WRITE when it happens and turning it off when it doesn't.
(d) You seem to be using one output buffer for all channels. You might be better off using a small one per channel. Then that way you don't clear, you just do put/flip/write/compact, and if the write returned 0 just post OP_WRITE for next time around the select loop. Then you won't lose any data at all, except to a client who really isn't reading: you can detect that situation by keeping track of the last successful write time to a channel, and when there is pending data and the last write is too long ago have a think about what the block means in terms of the application. Maybe you should just disconnect the client?
(e) It would be interesting to know how many times the write loop looped when you get these large delays, and also what the data size was, and also to know that for the other cases to see if there is a difference.
(f) Generally from a fairness point of view I prefer not to have write loops, just one attempt and if it returns even a short read I post OP_WRITE as above. Otherwise you're spending too long servicing one channel.
You can contact me offline via http://www.telekinesis.com.au if you like.

SocketChannel.write()

I've written a non-blocking server and am trying to test it with a client that uses the traditional (blocking) I/O. The problem I'm running into is when the server tries to write data to the client, it seems to take an inordinate amount of time.
Often times the SocketChannel.write() method will return zero and write nothing to the socket when it has a perfectly good ByteBuffer available to be written. The Javadoc for the SocketChannel.write() states this is normal:
Some types of channels, depending upon their state, may write only some of the bytes or possibly none at all. A socket channel in non-blocking mode, for example, cannot write any more bytes than are free in the socket's output buffer.
So I put the following statement in the code:
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While this solution delivers all of the data, it is slow. Has anybody run into this, and if so, found a better solution?
Thanks,
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I feel compelled to reply to myself to keep this alive, so to add:
It seems like the delays being experience here would not happen with blocking sockets and that this is a function of NIO. What I don't understand is why socket channel takes so long to know when it can write again. Waiting over half a second for one write of a 1000 byte message is not acceptable.
Is it because I am not using a Selector? Would that tell me the channel is ready any faster? I wouldn't see why. Or is it because the client application is reading the data too slowly?
Any help would be appreciated.
Ken

Does SocketChannel.write() block

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expected that an explicit cast would help.
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)pairArr;
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I'm doing; please trust me." But the compiler still
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Or, you could use pipelined function - i guess.
Like ->
satyaki>
satyaki>select * from v$version;
BANNER
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
Elapsed: 00:00:00.00
satyaki>
satyaki>create or replace type a_obj as object
2    (
3       e_nm varchar2(30),
4       e_sal number(7,2)
5    )
6 /
Type created.
Elapsed: 00:00:00.00
satyaki>
satyaki>
satyaki>create or replace type a_rec as table of a_obj;
2 /
Type created.
Elapsed: 00:00:00.00
satyaki>
satyaki>create or replace function multi_return(e_cd in number)
2 return a_rec
3 pipelined
4 is
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6    is
7      select a_obj(ename, sal) as a_det_rec
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10
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12 begin
13    for r1 in c1
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16    end loop;
17
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Elapsed: 00:00:00.01
satyaki>
satyaki>
satyaki>select * from table(cast(multi_return(&e_no) as a_rec));
Enter value for e_no: 7369
old   1: select * from table(cast(multi_return(&e_no) as a_rec))
new   1: select * from table(cast(multi_return(7369) as a_rec))
no rows selected
Elapsed: 00:00:00.02
satyaki>
satyaki>
satyaki>select * from emp;
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      7654 MARTIN     SALESMAN        7698 28-SEP-81       1815       1400         30 SALESMAN
      7788 SCOTT      ANALYST         7566 19-APR-87     598.95                    20 ANALYST
      7839 KING       PRESIDENT            17-NOV-81       7260                    10 PRESIDENT
      7844 TURNER     SALESMAN        7698 08-SEP-81       2178          0         30 SALESMAN
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old   1: select * from table(cast(multi_return(&e_no) as a_rec))
new   1: select * from table(cast(multi_return(7698) as a_rec))
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Glen                                 1848
Elapsed: 00:00:00.00
satyaki>
satyaki>/
Enter value for e_no: 7599
old   1: select * from table(cast(multi_return(&e_no) as a_rec))
new   1: select * from table(cast(multi_return(7599) as a_rec))
E_NM                                E_SAL
BILLY                                4500
Elapsed: 00:00:00.00
satyaki>Regards.
Satyaki De.

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Rizwan Ali Wahla wrote:
I want to know how can I write a cursor which told me no record exists before opening and running the loop.
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PS. Make sure that the SQL projection only includes the columns needed in PL/SQL. If the SQL is a pure exist row? check, then do not return any column and use a literal value as the projected column.
Edited by: Billy Verreynne on Feb 16, 2012 5:55 AM

Can SocketChannel.write return -1 ?

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