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33 <h1><a name="My_Project_"></a>Dalvik VM<br>Debug Monitor</h1>
35 <!-- Status is one of: Draft, Current, Needs Update, Obsolete -->
36 <p style="text-align:center"><strong>Status:</strong><em>Draft</em>
37 <small>(as of March 6, 2007)</small></p>
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61 <p>It's extremely useful to be able to monitor the live state of the
62 VM. For Android, we need to monitor multiple VMs running on a device
63 connected through USB or a wireless network connection. This document
64 describes a debug monitor server that interacts with multiple VMs, and
65 an API that VMs and applications can use to provide information
68 <p>Some things we can monitor with the Dalvik Debug Monitor ("DDM"):
70 <li> Thread states. Track thread creation/exit, busy/idle status.
71 <li> Overall heap status, useful for a heap bitmap display or
72 fragmentation analysis.
75 <p>It is possible for something other than a VM to act as a DDM client, but
76 that is a secondary goal. Examples include "logcat" log extraction
77 and system monitors for virtual memory usage and load average.
79 <p>It's also possible for the DDM server to be run on the device, with
80 the information presented through the device UI. However, the initial goal
81 is to provide a display tool that takes advantage of desktop tools and
84 <p>This work is necessary because we are unable to use standard JVMTI-based
85 tools with Dalvik. JVMTI relies on bytecode insertion, which is not
86 currently possible because Dalvik doesn't support Java bytecode.
88 <p>The DDM server is written in the Java programming language
89 for portability. It uses a desktop
90 UI toolkit (SWT) for its interface.
95 <p>To take advantage of existing infrastructure we are piggy-backing the
96 DDM protocol on top of JDWP (the Java Debug Wire Protocol, normally spoken
97 between a VM and a debugger). To a
98 non-DDM client, the DDM server just looks like a debugger.
100 <p>The JDWP protocol is very close to what we want to use. In particular:
102 <li>It explicitly allows for vendor-defined packets, so there is no
103 need to "bend" the JDWP spec.
104 <li>Events may be posted from the VM at arbitrary points. Such
105 events do not elicit a response from the debugger, meaning the client
106 can post data and immediately resume work without worrying about the
108 <li>The basic protocol is stateless and asynchronous. Request packets
109 from the debugger side include a serial number, which the VM includes
110 in the response packet. This allows multiple simultaneous
111 conversations, which means the DDM traffic can be interleaved with
115 <p>There are a few issues with using JDWP for our purposes:
117 <li>The VM only expects one connection from a debugger, so you couldn't
118 attach the monitor and a debugger at the same time. This will be
119 worked around by connecting the debugger to the monitor and passing the
120 traffic through. (We're already doing the pass-through with "jdwpspy";
121 requires some management of our request IDs though.) This should
122 be more convenient than the current "guess the port
123 number" system when we're attached to a device.
124 <li>The VM behaves differently when a debugger is attached. It will
125 run more slowly, and any objects passed to the monitor or debugger are
126 immune to GC. We can work around this by not enabling the slow path
127 until non-DDM traffic is observed. We also want to have a "debugger
128 has connected/disconnected" message that allows the VM to release
129 debugger-related resources without dropping the net connection.
130 <li>Non-DDM VMs should not freak out when DDM connects. There are
131 no guarantees here for 3rd-party VMs (e.g. a certain mainstream VM,
132 which crashes instantly), but our older JamVM can be
133 configured to reject the "hello" packet.
137 <h3>Connection Establishment</h3>
139 <p>There are two basic approaches: have the server contact the VMs, and
140 have the VMs contact the server. The former is less "precise" than the
141 latter, because you have to scan for the clients, but it has some
144 <p>There are three interesting scenarios:
146 <li>The DDM server is started, then the USB-attached device is booted
147 or the simulator is launched.
148 <li>The device or simulator is already running when the DDM server
150 <li>The DDM server is running when an already-started device is
153 <p>If we have the VMs connect to the DDM server on startup, we only handle
154 case #1. If the DDM server scans for VMs when it starts, we only handle
155 case #2. Neither handles case #3, which is probably the most important
156 of the bunch as the device matures.
157 <p>The plan is to have a drop-down menu with two entries,
158 "scan workstation" and "scan device".
159 The former causes the DDM server to search for VMs on "localhost", the
160 latter causes it to search for VMs on the other side of an ADB connection.
161 The DDM server will scan for VMs every few seconds, either checking a
162 range of known VM ports (e.g. 8000-8040) or interacting with some sort
163 of process database on the device. Changing modes causes all existing
164 connections to be dropped.
165 <p>When the DDM server first starts, it will try to execute "adb usb"
166 to ensure that the ADB server is running. (Note it will be necessary
167 to launch the DDM server from a shell with "adb" in the path.) If this
168 fails, talking to the device will still be possible so long as the ADB
169 daemon is already running.
171 <h4>Connecting a Debugger</h4>
173 <p>With the DDM server sitting on the JDWP port of all VMs, it will be
174 necessary to connect the debugger through the DDM server. Each VM being
175 debugged will have a separate port being listened to by the DDM server,
176 allowing you to connect a debugger to one or more VMs simultaneously.
178 <p>In the common case, however, the developer will only want to debug
179 a single VM. One port (say 8700) will be listened to by the DDM server,
180 and anything connecting to it will be connected to the "current VM"
181 (selected in the UI). This should allow developers to focus on a
182 single application, which may otherwise shift around in the ordering, without
183 having to adjust their IDE settings to a different port every time they
187 <h3>Packet Format</h3>
189 <p>Information is sent in chunks. Each chunk starts with:
194 and contains a variable amount of type-specific data.
195 Unrecognized types cause an empty response from the client and
196 are quietly ignored by the server. [Should probably return an error;
197 need an "error" chunk type and a handler on the server side.]
199 <p>The same chunk type may have different meanings when sent in different
200 directions. For example, the same type may be used for both a query and
201 a response to the query. For sanity the type must always be used in
202 related transactions.
204 <p>This is somewhat redundant with the JDWP framing, which includes a
205 4-byte length and a two-byte type code ("command set" and "command"; a
206 range of command set values is designated for "vendor-defined commands
207 and extensions"). Using the chunk format allows us to remain independent
208 of the underlying transport, avoids intrusive integration
209 with JDWP client code, and provides a way to send multiple chunks in a
210 single transmission unit. [I'm taking the multi-chunk packets into
211 account in the design, but do not plan to implement them unless the need
214 <p>Because we may be sending data over a slow USB link, the chunks may be
215 compressed. Compressed chunks are written as a chunk type that
216 indicates the compression, followed by the compressed length, followed
217 by the original chunk type and the uncompressed length. For zlib's deflate
218 algorithm, the chunk type is "ZLIB".
220 <p>Following the JDWP model, packets sent from the server to the client
221 are always acknowledged, but packets sent from client to server never are.
222 The JDWP error code field is always set to "no error"; failure responses
223 from specific requests must be encoded into the DDM messages.
225 <p>In what follows "u4" is an unsigned 32-bit value and "u1" is an
226 unsigned 8-bit value. Values are written in big-endian order to match
230 <h3>Initial Handshake</h3>
232 <p>After the JDWP handshake, the server sends a HELO chunk to the client.
233 If the client's JDWP layer rejects it, the server assumes that the client
234 is not a DDM-aware VM, and does not send it any further DDM queries.
235 <p>On the client side, upon seeing a HELO it can know that a DDM server
236 is attached and prepare accordingly. The VM should not assume that a
237 debugger is attached until a non-DDM packet arrives.
239 <h4>Chunk HELO (server --> client)</h4>
240 <p>Basic "hello" message.
242 u4 DDM server protocol version
246 <h4>Chunk HELO (client --> server, reply only)</h4>
247 Information about the client. Must be sent in response to the HELO message.
249 u4 DDM client protocol version
251 u4 VM ident string len (in 16-bit units)
252 u4 application name len (in 16-bit units)
253 var VM ident string (UTF-16)
254 var application name (UTF-16)
257 <p>If the client does not wish to speak to the DDM server, it should respond
258 with a JDWP error packet. This is the same behavior you'd get from a VM
259 that doesn't support DDM.
262 <h3>Debugger Management</h3>
263 <p>VMs usually prepare for debugging when a JDWP connection is established,
264 and release debugger-related resources when the connection drops. We want
265 to open the JDWP connection early and hold it open after the debugger
267 <p>The VM can tell when a debugger attaches, because it will start seeing
268 non-DDM JDWP traffic, but it can't identify the disconnect. For this reason,
269 we need to send a packet to the client when the debugger disconnects.
270 <p>If the DDM server is talking to a non-DDM-aware client, it will be
271 necessary to drop and re-establish the connection when the debugger goes away.
272 (This also works with DDM-aware clients; this packet is an optimization.)
274 <h4>Chunk DBGD (server --> client)</h4>
275 <p>Debugger has disconnected. The client responds with a DBGD to acknowledge
276 receipt. No data in request, no response required.
280 <p>Update the server's info about the client.
282 <h4>Chunk APNM (client --> server)</h4>
284 <p>If a VM's application name changes -- possible in our environment because
285 of the "pre-initialized" app processes -- it must send up one of these.
287 u4 application name len (in 16-bit chars)
288 var application name (UTF-16)
291 <h4>Chunk WAIT (client --> server)</h4>
293 <p>This tells DDMS that one or more threads are waiting on an external
294 event. The simplest use is to tell DDMS that the VM is waiting for a
297 u1 reason (0 = wait for debugger)
299 If DDMS is attached, the client VM sends this up when waitForDebugger()
300 is called. If waitForDebugger() is called before DDMS attaches, the WAIT
301 chunk will be sent up at about the same time as the HELO response.
304 <h3>Thread Status</h3>
306 <p>The client can send updates when their status changes, or periodically
307 send thread state info, e.g. 2x per
308 second to allow a "blinkenlights" display of thread activity.
310 <h4>Chunk THEN (server --> client)</h4>
312 <p>Enable thread creation/death notification.
314 u1 boolean (true=enable, false=disable)
316 <p>The response is empty. The client generates THCR packets for all
317 known threads. (Note the THCR packets may arrive before the THEN
320 <h4>Chunk THCR (client --> server)</h4>
321 <p>Thread Creation notification.
323 u4 VM-local thread ID (usually a small int)
324 u4 thread name len (in 16-bit chars)
325 var thread name (UTF-16)
328 <h4>Chunk THDE (client --> server)</h4>
329 <p>Thread Death notification.
331 u4 VM-local thread ID
334 <h4>Chunk THST (server --> client)</h4>
336 <p>Enable periodic thread activity updates.
337 Threads in THCR messages are assumed to be in the "initializing" state. A
338 THST message should follow closely on the heels of THCR.
342 <p>An interval of 0 disables the updates. This is done periodically,
343 rather than every time the thread state changes, to reduce the amount
344 of data that must be sent for an actively running VM.
346 <h4>Chunk THST (client --> server)</h4>
347 <p>Thread Status, describing the state of one or more threads. This is
348 most useful when creation/death notifications are enabled first. The
354 Then, for every thread:
356 u4 VM-local thread ID
360 <p>"thread state" must be one of:
361 <ul> <!-- don't use ol, we may need (-1) or sparse -->
362 <li> 1 - running (now executing or ready to do so)
363 <li> 2 - sleeping (in Thread.sleep())
364 <li> 3 - monitor (blocked on a monitor lock)
365 <li> 4 - waiting (in Object.wait())
366 <li> 5 - initializing
368 <li> 7 - native (executing native code)
369 <li> 8 - vmwait (waiting on a VM resource)
371 <p>"suspended" will be 0 if the thread is running, 1 if not.
372 <p>[Any reason not to make "suspended" be the high bit of "thread state"?
373 Do we need to differentiate suspend-by-GC from suspend-by-debugger?]
374 <p>[We might be able to send the currently-executing method. This is a
375 little risky in a running VM, and increases the size of the messages
376 considerably, but might be handy.]
381 <p>The client sends what amounts to a color-coded bitmap to the server,
382 indicating which stretches of memory are free and which are in use. For
383 compactness the bitmap is run-length encoded, and based on multi-byte
384 "allocation units" rather than byte counts.
386 <p>In the future the server will be able to correlate the bitmap with more
387 detailed object data, so enough information is provided to associate the
388 bitmap data with virtual addresses.
390 <p>Heaps may be broken into segments within the VM, and due to memory
391 constraints it may be desirable to send the bitmap in smaller pieces,
392 so the protocol allows the heap data to be sent in several chunks.
393 To avoid ambiguity, the client is required
394 to send explicit "start" and "end" messages during an update.
396 <p>All messages include a "heap ID" that can be used to differentiate
397 between multiple independent virtual heaps or perhaps a native heap. The
398 client is allowed to send information about different heaps simultaneously,
399 so all heap-specific information is tagged with a "heap ID".
401 <h4>Chunk HPIF (server --> client)</h4>
402 <p>Request heap info.
406 <p>The "when" values are:
414 <h4>Chunk HPIF (client --> server, reply only)</h4>
415 <p>Heap Info. General information about the heap, suitable for a summary
423 u8 timestamp in ms since Unix epoch
424 u1 capture reason (same as 'when' value from server)
425 u4 max heap size in bytes (-Xmx)
426 u4 current heap size in bytes
427 u4 current number of bytes allocated
428 u4 current number of objects allocated
430 <p>[We can get some of this from HPSG, more from HPSO.]
431 <p>[Do we need a "heap overhead" stat here, indicating how much goes to
432 waste? e.g. (8 bytes per object * number of objects)]
434 <h4>Chunk HPSG (server --> client)</h4>
435 <p>Request transmission of heap segment data.
440 <p>The "when" to send will be zero to disable transmission, 1 to send
441 during a GC. Other values are currently undefined. (Could use to pick
442 which part of the GC to send it, or cause periodic transmissions.)
443 <p>The "what" field is currently 0 for HPSG and 1 for HPSO.
444 <p>No reply is expected.
446 <h4>Chunk NHSG (server --> client)</h4>
447 <p>Request transmission of native heap segment data.
452 <p>The "when" to send will be zero to disable transmission, 1 to send
453 during a GC. Other values are currently undefined.
454 <p>The "what" field is currently ignored.
455 <p>No reply is expected.
457 <h4>Chunk HPST/NHST (client --> server)</h4>
458 <p>This is a Heap Start message. It tells the server to discard any
459 existing notion of what the client's heap looks like, and prepare for
460 new information. HPST indicates a virtual heap dump and must be followed
461 by zero or more HPSG/HPSO messages and an HPEN. NHST indicates a native
462 heap dump and must be followed by zero or more NHSG messages and an NHEN.
464 <p>The only data item is:
469 <h4>Chunk HPEN/NHEN (client --> server)</h4>
470 <p>Heap End, indicating that all information about the heap has been sent.
471 A HPST will be paired with an HPEN and an NHST will be paired with an NHEN.
473 <p>The only data item is:
478 <h4>Chunk HPSG (client --> server)</h4>
479 <p>Heap segment data. Each chunk describes all or part of a contiguous
480 stretch of heap memory.
483 u1 size of allocation unit, in bytes (e.g. 8 bytes)
484 u4 virtual address of segment start
485 u4 offset of this piece (relative to the virtual address)
486 u4 length of piece, in allocation units
489 <p>The "usage data" indicates the status of each allocation unit. The data
490 is a stream of pairs of bytes, where the first byte indicates the state
491 of the allocation unit, and the second byte indicates the number of
492 consecutive allocation units with the same state.
493 <p>The bits in the "state" byte have the following meaning:
495 +---------------------------------------+
496 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
497 +---------------------------------------+
498 | P | U0 | K2 | K1 | K0 | S2 | S1 | S0 |
499 +---------------------------------------+
505 <li>1=has hard reference
506 <li>2=has soft reference
507 <li>3=has weak reference
508 <li>4=has phantom reference
509 <li>5=pending finalization
510 <li>6=marked, about to be swept
516 <li>2=array of byte/boolean
517 <li>3=array of char/short
518 <li>4=array of Object/int/float
519 <li>5=array of long/double
521 <li>'P': partial flag (not used for HPSG)
522 <li>'U': unused, must be zero
525 <p>The use of the various 'S' types depends on when the information is
526 sent. The current plan is to send it either immediately after a GC,
527 or between the "mark" and "sweep" phases of the GC. For a fancy generational
528 collector, we may just want to send it up periodically.
530 <p>The run-length byte indicates the number of allocation units minus one, so a
531 length of 255 means there are 256 consecutive units with this state. In
532 some cases, e.g. arrays of bytes, the actual size of the data is rounded
533 up the nearest allocation unit.
534 <p>For HPSG, the runs do not end at object boundaries. It is not possible
535 to tell from this bitmap whether a run contains one or several objects.
536 (But see HPSO, below.)
537 <p>[If we find that we have many long runs, we can overload the 'P' flag
538 or dedicate the 'U' flag to indicate that we have a 16-bit length instead
539 of 8-bit. We can also use a variable-width integer scheme for the length,
540 encoding 1-128 in one byte, 1-16384 in two bytes, etc.]
541 <p>[Alternate plan for 'K': array of byte, array of char, array of Object,
542 array of miscellaneous primitive type]
543 <p>To parse the data, the server runs through the usage data until either
544 (a) the end of the chunk is reached, or (b) all allocation units have been
545 accounted for. (If these two things don't happen at the same time, the
547 <p>Example: suppose a VM has a heap at 0x10000 that is 0x2000 bytes long
548 (with an 8-byte allocation unit size, that's 0x0400 units long).
549 The client could send one chunk (allocSize=8, virtAddr=0x10000, offset=0,
550 length=0x0400) or two (allocSize=8, virtAddr=0x10000, offset=0, length=0x300;
551 then allocSize=8, virtAddr=0x10000, offset=0x300, length=0x100).
552 <p>The client must encode the entire heap, including all free space at
553 the end, or the server will not have an accurate impression of the amount
554 of memory in the heap. This refers to the current heap size, not the
557 <h4>Chunk HPSO (client --> server)</h4>
558 <p>This is essentially identical to HPSG, but the runs are terminated at
559 object boundaries. If an object is larger than 256 allocation units, the
560 "partial" flag is set in all runs except the last.
561 <p>The resulting unpacked bitmap is identical, but the object boundary
562 information can be used to gain insights into heap layout.
563 <p>[Do we want to have a separate message for this? Maybe just include
564 a "variant" flag in the HPST packet. Another possible form of output
565 would be one that indicates the age, in generations, of each block of
566 memory. That would provide a quick visual indication of "permanent vs.
567 transient residents", perhaps with a 16-level grey scale.]
569 <h4>Chunk NHSG (client --> server)</h4>
570 <p>Native heap segment data. Each chunk describes all or part of a
571 contiguous stretch of native heap memory. The format is the same as
572 for HPSG, except that only solidity values 0 (= free) and 1 (= hard
573 reference) are used, and the kind value is always 0 for free chunks
574 and 7 for allocated chunks, indicating a non-VM object.
577 u1 size of allocation unit, in bytes (e.g. 8 bytes)
578 u4 virtual address of segment start
579 u4 offset of this piece (relative to the virtual address)
580 u4 length of piece, in allocation units
584 <h3>Generic Replies</h3>
586 The client-side chunk handlers need a common way to report simple success
587 or failure. By convention, an empty reply packet indicates success.
589 <h4>Chunk FAIL (client --> server, reply only)</h4>
590 <p>The chunk includes a machine-readable error code and a
591 human-readable error message. Server code can associate the failure
592 with the original request by comparing the JDWP packet ID.
593 <p>This allows a standard way of, for example, rejecting badly-formed
597 u4 error message len (in 16-bit chars)
598 var error message (UTF-16)
601 <h3>Miscellaneous</h3>
603 <h4>Chunk EXIT (server --> client)</h4>
604 <p>Cause the client to exit with the specified status, using System.exit().
605 Useful for certain kinds of testing.
610 <h4>Chunk DTRC (server --> client)</h4>
611 <p>[TBD] start/stop dmtrace; can send the results back over the wire. For
612 size reasons we probably need "sending", "data", "key", "finished" as
613 4 separate chunks/packets rather than one glob.
618 <p>The API is written in the Java programming language
619 for convenience. The code is free to call native methods if appropriate.
621 <h3>Chunk Handler API</h3>
623 <p>The basic idea is that arbitrary code can register handlers for
624 specific chunk types. When a DDM chunk with that type arrives, the
625 appropriate handler is invoked. The handler's return value provides the
626 response to the server.
628 <p>There are two packages. android.ddm lives in the "framework" library,
629 and has all of the chunk handlers and registration code. It can freely
630 use Android classes. org.apache.harmony.dalvik.ddmc lives in the "core"
632 some base classes and features that interact with the VM. Nothing should
633 need to modify the org.apache.harmony.dalvik.ddmc classes.
635 <p>The DDM classes pass chunks of data around with a simple class:
637 <pre class=prettyprint>
645 <p>The chunk handlers accept and return them:
646 <pre class=prettyprint>
647 public Chunk handleChunk(Chunk request)
649 <p>The code is free to parse the chunk and generate a response in any
650 way it chooses. Big-endian byte ordering is recommended but not mandatory.
651 <p>Chunk handlers will be notified when a DDM server connects or disconnects,
652 so that they can perform setup and cleanup operations:
653 <pre class=prettyprint>
654 public void connected()
655 public void disconnected()
658 <p>The method processes the request, formulates a response, and returns it.
659 If the method returns null, an empty JDWP success message will be returned.
660 <p>The request/response interaction is essentially asynchronous in the
661 protocol. The packets are linked together with the JDWP message ID.
662 <p>[We could use ByteBuffer here instead of byte[], but it doesn't gain
663 us much. Wrapping a ByteBuffer around an array is easy. We don't want
664 to pass the full packet in because we could have multiple chunks in one
665 request packet. The DDM code needs to collect and aggregate the responses
666 to all chunks into a single JDWP response packet. Parties wanting to
667 write multiple chunks in response to a single chunk should send a null
668 response back and use "sendChunk()" to send the data independently.]
670 <h3>Unsolicited event API</h3>
672 <p>If a piece of code wants to send a chunk of data to the server at some
673 arbitrary time, it may do so with a method provided by
674 org.apache.harmony.dalvik.DdmServer:
676 <pre class=prettyprint>
677 public static void sendChunk(Chunk chunk)
680 <p>There is no response or status code. No exceptions are thrown.
685 <p>This is similar to the client side in many ways, but makes extensive
686 use of ByteBuffer in a perhaps misguided attempt to use java.nio.channels
687 and avoid excessive thread creation and unnecessary data copying.
689 <p>Upon receipt of a packet, the server will identify it as one of:
691 <li>Message to be passed through to the debugger
692 <li>Response to an earlier request
693 <li>Unsolicited event packet
695 <p>To handle (2), when messages are sent from the server to the client,
696 the message must be paired with a callback method. The response might be
697 delayed for a while -- or might never arrive -- so the server can't block
698 waiting for responses from the client.
699 <p>The chunk handlers look like this:
700 <pre class=prettyprint>
701 public void handleChunk(Client client, int type,
702 ByteBuffer data, boolean isReply, int msgId)
704 <p>The arguments are:
707 <dd>An object representing the client VM that send us the packet.
709 <dd>The 32-bit chunk type.
711 <dd>The data. The data's length can be determined by calling data.limit().
713 <dd>Set to "true" if this was a reply to a message we sent earlier,
714 "false" if the client sent this unsolicited.
716 <dd>The JDWP message ID. Useful for connecting replies with requests.
718 <p>If a handler doesn't like the contents of a packet, it should log an
719 error message and return. If the handler doesn't recognize the packet at
720 all, it can call the superclass' handleUnknownChunk() method.
722 <p>As with the client, the server code can be notified when clients
723 connect or disconnect. This allows the handler to send initialization
724 code immediately after a connect, or clean up after a disconnect.
725 <p>Data associated with a client can be stored in a ClientData object,
726 which acts as a general per-client dumping around for VM and UI state.
733 <address>Copyright © 2007 The Android Open Source Project</address>