Finally got around to reading up on this. At first I was expecting to see it originally sent one large packet, but from reading up on the issue it looks like the problem was it sent a packet for each field of the JDWP header, resulting in too many small packets, and the fix for this was to coalesce the header into one packet. So this CR doesn't explain why two packets are sent (when the data is large) instead of one.


On 2/26/18 8:20 PM, David Holmes wrote:
The two-step send came in with:

"Small JDWP packets with the socket transport causes slow debugging on linux 2.6.15 kernel and newer"


On 27/02/2018 9:29 AM, wrote:
On 2/26/18 15:06, Chris Plummer wrote:
On 2/26/18 3:00 PM, wrote:

On 2/26/18 12:16 PM, Chris Plummer wrote:
On 2/26/18 11:51 AM, wrote:
Hi David and Sergei,

On 2/20/18 10:16 PM, wrote:
Hi David,

On 2/20/18 20:02, David Holmes wrote:
Hi Daniil,

Good find on this!

What does the actual spec say about the length of things and how they may be split across multiple packets? Are we guaranteed that at most two packets will be involved?

The JDWP spec ( says nothing about splitting JDWP reply packets at all but the implementation limits the max number of the sent packets to two packets max. The implementation is dated back to the initial load that happened in 2007 and the information about the related Jira issue is missing.


836    data = packet->;
837    /* Do one send for short packets, two for longer ones */
838    if (data_len <= MAX_DATA_SIZE) {
839        memcpy(header + JDWP_HEADER_SIZE, data, data_len);
840        if (send_fully(socketFD, (char *)&header, JDWP_HEADER_SIZE + data_len) !=
841            JDWP_HEADER_SIZE + data_len) {
842            RETURN_IO_ERROR("send failed");
843        }
844    } else {
845        memcpy(header + JDWP_HEADER_SIZE, data, MAX_DATA_SIZE);
846        if (send_fully(socketFD, (char *)&header, JDWP_HEADER_SIZE + MAX_DATA_SIZE) !=
847            JDWP_HEADER_SIZE + MAX_DATA_SIZE) {
848            RETURN_IO_ERROR("send failed");
849        }
850        /* Send the remaining data bytes right out of the data area. */
851        if (send_fully(socketFD, (char *)data + MAX_DATA_SIZE,
852                       data_len - MAX_DATA_SIZE) != data_len - MAX_DATA_SIZE) {
853            RETURN_IO_ERROR("send failed");
854        }
855    }

Curious. First packet is limited to MAX_DATA_SIZE, 2nd packet has no size limit. What's the point then of splitting it then? Is there a desire to get the header transmitted in a smaller packet.


It looks as the goal was to somehow improve the responsiveness in case of the large data but I am not sure about this. I could not locate any traces in Jira related to this implementation.
I was thinking it might be something like that too. Get the header across the wire quickly. Maybe the user just wants the header (with size info) initially, and will allocate a large buffer for the rest if necessary.

It was my guess too.
At least, it is the best explanation for this design that looks reasonable to me.

Probably Serguei has some info what is the history behind this design.

I don't know the history here.
This was implemented in very early days, most likely, before JDK 1.5 or even 1.4.2.


 68     protected byte[] readJdwpString(DataInputStream ds) throws IOException {
  69         byte[] str = null;
  70         int len = ds.readInt();
  71         if (len > 0) {
  72             str = new byte[len];
  73   , 0, len);
  74         }

might we get a short-read of the string if it is split across multiple packets?

This and all other reads happen not directly from the socket input stream but rather from the  DataInputStream object that is constructed in JdwpReply.initFromStream(InputStream) method. With the proposed fix we do ensure that the created DataInputStream object contains data from both packets in cases when the reply was split in two packets.

Nice catch!
Even though this fix is enough to resolve this problem now, there is a chance, it can fail in the future when more modules are added to the platform.

I'm wondering if all these reads should be loops, ensuring we read the expected amount of data.

Since the implementation of the socket transport limits the max number of packets the reply might be split in to two packets I don't think we really need it here.
One further comment - not sure why we need the print out for when we do read multiple packets?
That would seem to be a debugging aid.

Yes, it helps to understand what happens.
Many tests have a lack of tracing which makes it harder to debug and understand failures.
That is correct.  This additional tracing was added to help to understand the possible failures in the future.




On 21/02/2018 10:14 AM, Daniil Titov wrote:
Hi Serguei,

A new version of the webrev that has these strings reformatted is at

Thank you!

Best regards,


*From: *"" <>
*Date: *Tuesday, February 20, 2018 at 3:00 PM
*To: *Daniil Titov <>, "" <> *Subject: *Re: RFR 8170541: serviceability/jdwp/ fails intermittently on Windows and Solaris

Hi Daniil,

Interesting issue...
Thank you for finding to the root cause so quickly!

The fix looks good.
Could I ask you to reformat these lines to make the L54 shorter ?:

   54                 System.out.println("[" + getClass().getName() + "] Only " + bytesRead + " bytes of " + dataLength +

   55                         " were read in the first packet. Reading the rest...");


On 2/20/18 09:24, Daniil Titov wrote:

    Please review the changes that fix intermittent failure of
    serviceability/jdwp/ test.

    The problem here is that for a large data the JDWP agent
    (socketTransport_writePacket() method in
src/jdk.jdwp.agent/share/native/libdt_socket/socketTransport.c )
    sends 2 packets and in some cases only the first packet is received     at the time when the test reads the reply from the JDWP agent. Since     the test does not check that all data is received in the first     packet the correlation between commands and replies became broken     (the unread second packet is read by the next command and the reply     for the next command is read by the next after next command and so on).



    The tests ran successfully with Mach5.

    Best regards,


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