[lldb-dev] Is there support for command queuing/asynchronicity in lldb?

Kuba Ober kuba at mareimbrium.org
Fri Mar 14 13:58:42 PDT 2014

I haven’t noticed it before, but obviously there already is an api for event notifications,
so that part (arguably, a hard part), doesn’t have to be reinvented! This would serve
perfectly for notifications about commands that have completed. About the only missing part
is having a way of passing “variant” data through an event.

I fully agree that there should be full support for entirely synchronous way to use the api,
since for local use this makes the most sense, and lot of third-party code depends
on that.

There are two reasons for command queuing and asynchronous execution. Interestingly enough,
both slow and fast remote targets benefit from it. By “slow” and “fast” I mean the speed
of the “dongle” used to connect to a target (whatever attaches to the chip’s OCD,
could be serial, JTAG, etc.)

1. On slow targets that take long to process some commands, shorter commands could be interspersed
with the long ones to give better feel to the user. This is especially important when a full
memory dump is needed upon startup (say at the implicit breakpoint in `main` entry point).
I have a target where a memory dump takes 7 seconds, and you need one every time you stop
on a breakpoint.

2. On fast targets, the latency of the lldb code itself, and the user code, is such, that
dongle’s bandwidth is wasted. Since dongle bandwidth costs real money (fast dongles cost more),
it’s a real waste not to translate all that expenditure into smooth UI behavior for the user.

Note that the *user* (such as an IDE) is not really in a position to make decisions such as
memory read chunk size. Sure, we could offer an API that suggests a chunk size, but there are
potentially many other operations that simply take long and maybe not even have anything
like a chunk size - they atomically are long, yet shouldn’t hang up the entire debugger.

It seems that the things that are worth asynchronous calling already provide SBError result.

A simple async-ification would:

1. Add an optional SBAsync & async = SBAsync::Null() parameter at the end of the parameter
list. This class would have fields that determine the following:
- that the command is to execute asynchronously,
- a client-provided command ID to pass into the result,
- the ordering behavior (if any),
- whether the result is to be provided via an event broadcast (and what is the listener).

2. Amend SBError to provide an InProgress() state, and a way to get the copy of SBAsync
instance that was used to start the command (to correlate with command ID etc),
and a way of getting whatever data was requested. Such result would be asynchronously
updated as the command progresses, allowing to poll progress. 

Some APIs return an error code instead of using SBError, so those would need to be amended,
to do both. For example
SBProcess::GetProcessId() -> SBProcess::GetProcessId(SBError & err = SBError());

Does this make any sense?

Cheers, Kuba Ober

On Mar 14, 2014, at 2:56 PM, jingham at apple.com wrote:

> My take on this would be to keep the SB API's as simple as possible, and then add a library on top of the SB API's that would implement some flavor of asynchronous operation.  After all, lots of folks have opinions about how this sort of thing should be done, arguments which the core SB API's should stand to the side of if at all possible...
> If there is anything about supporting various models of async execution that would need support not currently available through the SB API's as they now stand, then we should provide that.  That would keep us honest about supporting different flavors of using our API, and also keep orthogonal complexity out of the core of the debugger.  For instance, the SB API's don't currently have a way to say "I am going to be doing a whole bunch of operations, and I don't want anybody to be able to restart the process while that is going on."  That is a primitive that would be needed for async operation to be reliably coded up, and Greg and I have talked about ways to do this for quite some time, but haven't had time to code anything up yet.  But again, I'd much rather provide this sort of thing, and then be agnostic about how clients want to use our API's. 
> Jim
> On Mar 14, 2014, at 10:30 AM, Greg Clayton <gclayton at apple.com> wrote:
>> On Mar 14, 2014, at 9:46 AM, Kuba Ober <kuba at mareimbrium.org> wrote:
>>> Do lldb internals support/make it possible to queue commands
>>> for remote debugging, and to have the results of the same
>>> reported asynchronously?
>> No, not currently.
>>> This is needed for good user experience on targets that
>>> are being debugged remotely or via JTAG and similar debugging
>>> dongles. Quite often such connections are slow and the
>>> commands can take very long to execute.
>>> Ideally, the user or the IDE should not be expected to
>>> know to split long-running commands into smaller pieces/
>>> Specifically, I’m thinking of following features:
>>> 1. Submit a command for execution asynchronously -
>>> it returns right away, and the result(s) are reported lated.
>>> 2. Get partial results from asynchronous command execution
>>> as it progresses. For example, during a long memory read
>>> it’d be nice to get periodic notifications as each “chunk”
>>> of data comes in.
>>> 3. Specification of partial ordering between commands.
>>> The default would be to have totally ordered command execution
>>> as is the case right now, but sometimes this can be relaxed.
>>> Again, think of a very long running memory dump - several
>>> megabytes of stuff being read, it can take dozens of seconds
>>> on slow dongles or slow network connections. If the user
>>> (or an IDE) wants, the subsequent commands can be given
>>> with relaxed ordering such that they don’t have to be delayed
>>> until the memory read finishes. Say that a user wants to change
>>> a register while the memory is dumped, or request a smaller
>>> read somewhere else that could finish much sooner.
>>> This would provide for good interactive user experience in IDEs.
>>> Any thoughts/hints/input?
>> I don't believe async should be built into our API as this would really change the entire nature of the API. It would also seriously affect our Python API bindings and all existing programs that currently link the LLDB.
>> But that isn't to say we can add a "read/write memory in background" to our Process API. That would be really useful and it would be great to be able to cancel such an operation. So I would vote to identify these long running operations and add support for them into our public API.
>> So we could add something like this to SBProcess:
>> class SBProcess {
>>   typedef bool (*MemoryProgressCallback)(
>> 	addr_t orig_addr, // Original start address for memory read/write
>> 	addr_t orig_size, // Original size of the memory read/write
>> 	addr_t curr_pos,  // The address that was just written used for progress
>>       SBError &error,   // The error for the current memory read/write
>> 	void* baton);	  // The void * passed to the read/write memory in background call
>>   void
>>   ReadMemoryInBackground (addr_t addr, void *buf, size_t size, lldb::SBError &error, MemoryProgressCallback callback, void *baton);
>>   void
>>   WriteMemoryInBackground (addr_t addr, const void *buf, size_t size, lldb::SBError &error, MemoryProgressCallback callback, void *baton);
>> }
>> When the MemoryProgressCallback is called, it can return "true" to keep going, for "false" to cancel.
>> So I would vote:
>> 1 - identify things we know are going to take a long time and make sure we have an API for common things (like long read/write memory)
>> 2 - Use the API to build things that aren't commonly needed by everyone.
>> Right now, an IDE can implement the background memory read/write with progress using our public API. Just spin off another thread and break the read/write up yourself and submit smaller reads/writes. Have your other thread run your GUI and it will be able to read/write registers, etc without interrupting your memory read/write thread.
>> Our API is multi-threaded aware and safe, so there might not even be anything we really need to do here. Just use the existing API. The memory read/write in the background would be a nice addition to the API, so I would be happy to have this added as a nice functionality so not everyone has to code this up themselves.
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