Re: [lldb-dev] Breakpoint + callback performance ... Can it be faster?

[Benjamin Dicken via lldb-dev]

On Tue, Aug 16, 2016 at 11:06 AM, Jim Ingham  wrote:

>
> > On Aug 16, 2016, at 10:42 AM, Benjamin Dicken <
> bddic...@datawareventures.com> wrote:
> >
> > Thanks for the quick reply.
> >
> > > Are you sure the actual handling of the breakpoint & callback in lldb
> is what is taking most of the time?
> >
> > I'm not positive. I did collect some callgrind profiles to take a look
> at where most of the time is being spent, but i'm not very familiar with
> lldb internals so the results were hard to interpret. I did notice that
> there was a lot of packet/network business when using lldb to profile a
> program (which I assumed was communication between my program and
> lldb-server). I was not sure how this effected the performance, so perhaps
> this is the real bottleneck.
>
> I would be pretty surprised if it was not.  We had some bugs in breakpoint
> handling - mostly related to having very very many breakpoints.  But other
> than that the dispatching of the breakpoint StopInfo is a pretty simple,
> straight forward bit of work.
>
> >
> > > Greg just switched to using a unix-domain socket for this
> communication for platforms that support it.  This speeds up the packet
> traffic side of things.
> >
> > In what version of lldb was this introduced? I'm running 3.7.1. I'm also
> on ubuntu 14.04, is that a supported platform?
>
> It is just in TOT lldb, he just added it last week.  It is currently only
> turned on for OS X.
>

Good to know, thanks.

>
> >
> > > One of the original motivations of having lldb-server be based on lldb
> classes - as opposed to the MacOS X version of debugserver which is an
> independent construct - was that you could re-use the server code to create
> an in-process Process plugin, eliminating a lot of this traffic & context
> switching when you needed maximum speed.
> >
> > That sounds very interesting. Is there an example of this implementation
> you could point me to?
> >
>
> FreeBSB & Windows still have native Process plugins.  But they aren't used
> for the lldb-server implementation so far as I can tell (I've mostly worked
> on the OS X side.)  I think this was more of a design intent that hasn't
> actually been used anywhere yet.  But the Linux/Android folks will know
> better.
>

If any of the Linux/Andriod folks do know, please get in touch with me.
Thanks,


> Jim
>
>
> >
> >
> > On Tue, Aug 16, 2016 at 10:20 AM, Jim Ingham  wrote:
> > Are you sure the actual handling of the breakpoint & callback in lldb is
> what is taking most of the time?  The last time we looked at this, the
> majority of the work was in communicating with debugserver to get the stop
> notification and restart.  Note, besides all the packet code, this involves
> context switches from process->lldbserver->lldb and back, which is also
> pretty expensive.
> >
> > Greg just switched to using a unix-domain socket for this communication
> for platforms that support it.  This speeds up the packet traffic side of
> things.
> >
> > One of the original motivations of having lldb-server be based on lldb
> classes - as opposed to the MacOS X version of debugserver which is an
> independent construct - was that you could re-use the server code to create
> an in-process Process plugin, eliminating a lot of this traffic & context
> switching when you needed maximum speed.  The original Mac OS X lldb port
> actually had a process plugin wholly in-process with lldb as well as the
> debugserver based one, but there wasn't enough motivation to justify
> maintaining the two different implementations of the same code.  I don't
> know whether the Linux port takes advantage of this possibility, however.
> That would be something to look into, however.
> >
> > Once we actually figure out about the stop, figuring out the breakpoint
> and getting to its callback is pretty simple...  I doubt making "lighter
> weight breakpoints" in particular will recover the performance you need,
> though if your sampling turns up some inefficient algorithms have crept in,
> it would be great to fix that.
> >
> > Another option we've toyed with on and off is something like the gdb
> "tracepoints" were you can upload instructions to perform "experiments"
> when a breakpoint is hit to the lldb-server instance.  The work to perform
> the experiment and the results would all be kept in the lldb-server
> instance till a real breakpoint is hit, at which point lldb can download
> all the results and present them to the user.  This would eliminate some of
> the context-switches and packet traffic while you were running in the hot
> parts of your code.  This is a decent chunk of work, however.
> >
> > Jim
> >
> >
> > > On Aug 16, 2016, at 9:57 AM, Benjamin Dicken via lldb-dev <
> lldb-dev@lists.llvm.org> wrote:
> > >
> > > I recently started using lldb to write a basic instrumentation tool
> for tracking the values of variables at various code-points in a program.
> I've been working with lldb for less than two 

Re: [lldb-dev] Search C++ "virtual" objects

[Greg Clayton via lldb-dev]

> On Aug 15, 2016, at 4:59 PM, Lei Kong  wrote:
> 
> Should I use lldb::SBTarget::GetModuleAtIndex() to get all modules and then 
> lldb::SBModule::GetSymbolAtIndex() to go through all symbols?
> Thanks.

This is the only way right now as we don't have a SBModule::FindSymbols that 
takes a regular expression. We only currently have:

lldb::SBSymbolContextList
FindSymbols (const char *name,
 lldb::SymbolType type = eSymbolTypeAny);


If you want to, you can add a:


lldb::SBSymbolContextList
FindSymbolsByRegex (const char *regex, lldb::SymbolType type = 
eSymbolTypeAny);


Then you could call this:


SBModule module = ...;
lldb::SBSymbolContextList symbols = module.FindSymbolsByRegex("^vtable for ");


You can then also add this to SBTarget. Let me know if you are interested in 
adding this and I can help you do it.

Greg Clayton


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Re: [lldb-dev] Breakpoint + callback performance ... Can it be faster?

[Benjamin Dicken via lldb-dev]

Thanks for the quick reply.

> Are you sure the actual handling of the breakpoint & callback in lldb is
what is taking most of the time?

I'm not positive. I did collect some callgrind profiles to take a look at
where most of the time is being spent, but i'm not very familiar with lldb
internals so the results were hard to interpret. I did notice that there
was a lot of packet/network business when using lldb to profile a program
(which I assumed was communication between my program and lldb-server). I
was not sure how this effected the performance, so perhaps this is the real
bottleneck.

> Greg just switched to using a unix-domain socket for this communication
for platforms that support it.  This speeds up the packet traffic side of
things.

In what version of lldb was this introduced? I'm running 3.7.1. I'm also on
ubuntu 14.04, is that a supported platform?

> One of the original motivations of having lldb-server be based on lldb
classes - as opposed to the MacOS X version of debugserver which is an
independent construct - was that you could re-use the server code to create
an in-process Process plugin, eliminating a lot of this traffic & context
switching when you needed maximum speed.

That sounds very interesting. Is there an example of this implementation
you could point me to?



On Tue, Aug 16, 2016 at 10:20 AM, Jim Ingham  wrote:

> Are you sure the actual handling of the breakpoint & callback in lldb is
> what is taking most of the time?  The last time we looked at this, the
> majority of the work was in communicating with debugserver to get the stop
> notification and restart.  Note, besides all the packet code, this involves
> context switches from process->lldbserver->lldb and back, which is also
> pretty expensive.
>
> Greg just switched to using a unix-domain socket for this communication
> for platforms that support it.  This speeds up the packet traffic side of
> things.
>
> One of the original motivations of having lldb-server be based on lldb
> classes - as opposed to the MacOS X version of debugserver which is an
> independent construct - was that you could re-use the server code to create
> an in-process Process plugin, eliminating a lot of this traffic & context
> switching when you needed maximum speed.  The original Mac OS X lldb port
> actually had a process plugin wholly in-process with lldb as well as the
> debugserver based one, but there wasn't enough motivation to justify
> maintaining the two different implementations of the same code.  I don't
> know whether the Linux port takes advantage of this possibility, however.
> That would be something to look into, however.
>
> Once we actually figure out about the stop, figuring out the breakpoint
> and getting to its callback is pretty simple...  I doubt making "lighter
> weight breakpoints" in particular will recover the performance you need,
> though if your sampling turns up some inefficient algorithms have crept in,
> it would be great to fix that.
>
> Another option we've toyed with on and off is something like the gdb
> "tracepoints" were you can upload instructions to perform "experiments"
> when a breakpoint is hit to the lldb-server instance.  The work to perform
> the experiment and the results would all be kept in the lldb-server
> instance till a real breakpoint is hit, at which point lldb can download
> all the results and present them to the user.  This would eliminate some of
> the context-switches and packet traffic while you were running in the hot
> parts of your code.  This is a decent chunk of work, however.
>
> Jim
>
>
> > On Aug 16, 2016, at 9:57 AM, Benjamin Dicken via lldb-dev <
> lldb-dev@lists.llvm.org> wrote:
> >
> > I recently started using lldb to write a basic instrumentation tool for
> tracking the values of variables at various code-points in a program. I've
> been working with lldb for less than two weeks, so I am pretty new. Though,
> I have used and written llvm passes in the past, so I'm familiar with the
> clang/llvm/lldb ecosystem.
> >
> > I have a very early prototype of the tool up and running, using the C++
> API. The user can specify either an executable to run or an already-running
> PID to attach to. The user also supplies a file+line_number at which a
> breakpoint (with a callback) is placed. For testing/prototyping purposes,
> the breakpoint callback just increments a counter and then immediately
> returns false. Eventually, more interesting things will happen in this
> callback.
> >
> > I've noticed that just the action of hitting a breakpoint and invoking
> the callback is very expensive. I did some instruction-count collection by
> running this lldb tool on a simple test program, and placing the
> breakpoint+callback at different points in the program, causing it to get
> triggered different amounts of times. I used `perf stat -e instructions
> ...` to gather instruction exec counts for each run. After doing a little
> math, it appears that I'm incurring 1.0 - 

Re: [lldb-dev] Breakpoint + callback performance ... Can it be faster?

[Jim Ingham via lldb-dev]

Are you sure the actual handling of the breakpoint & callback in lldb is what 
is taking most of the time?  The last time we looked at this, the majority of 
the work was in communicating with debugserver to get the stop notification and 
restart.  Note, besides all the packet code, this involves context switches 
from process->lldbserver->lldb and back, which is also pretty expensive.  

Greg just switched to using a unix-domain socket for this communication for 
platforms that support it.  This speeds up the packet traffic side of things.  

One of the original motivations of having lldb-server be based on lldb classes 
- as opposed to the MacOS X version of debugserver which is an independent 
construct - was that you could re-use the server code to create an in-process 
Process plugin, eliminating a lot of this traffic & context switching when you 
needed maximum speed.  The original Mac OS X lldb port actually had a process 
plugin wholly in-process with lldb as well as the debugserver based one, but 
there wasn't enough motivation to justify maintaining the two different 
implementations of the same code.  I don't know whether the Linux port takes 
advantage of this possibility, however.  That would be something to look into, 
however.

Once we actually figure out about the stop, figuring out the breakpoint and 
getting to its callback is pretty simple...  I doubt making "lighter weight 
breakpoints" in particular will recover the performance you need, though if 
your sampling turns up some inefficient algorithms have crept in, it would be 
great to fix that.

Another option we've toyed with on and off is something like the gdb 
"tracepoints" were you can upload instructions to perform "experiments" when a 
breakpoint is hit to the lldb-server instance.  The work to perform the 
experiment and the results would all be kept in the lldb-server instance till a 
real breakpoint is hit, at which point lldb can download all the results and 
present them to the user.  This would eliminate some of the context-switches 
and packet traffic while you were running in the hot parts of your code.  This 
is a decent chunk of work, however.

Jim


> On Aug 16, 2016, at 9:57 AM, Benjamin Dicken via lldb-dev 
>  wrote:
> 
> I recently started using lldb to write a basic instrumentation tool for 
> tracking the values of variables at various code-points in a program. I've 
> been working with lldb for less than two weeks, so I am pretty new. Though, I 
> have used and written llvm passes in the past, so I'm familiar with the 
> clang/llvm/lldb ecosystem.
> 
> I have a very early prototype of the tool up and running, using the C++ API. 
> The user can specify either an executable to run or an already-running PID to 
> attach to. The user also supplies a file+line_number at which a breakpoint 
> (with a callback) is placed. For testing/prototyping purposes, the breakpoint 
> callback just increments a counter and then immediately returns false. 
> Eventually, more interesting things will happen in this callback.
> 
> I've noticed that just the action of hitting a breakpoint and invoking the 
> callback is very expensive. I did some instruction-count collection by 
> running this lldb tool on a simple test program, and placing the 
> breakpoint+callback at different points in the program, causing it to get 
> triggered different amounts of times. I used `perf stat -e instructions ...` 
> to gather instruction exec counts for each run. After doing a little math, it 
> appears that I'm incurring 1.0 - 1.1 million instruction execs per breakpoint.
> 
> This amount of slowdown is prohibitively expensive for my needs, because I 
> want to place callbacks in hot portions of the "inferior" program.
> 
> Is there a way to make this faster? Is it possible to create "lighter-weight" 
> breakpoints? I really like the lldb API (though the documentation is lacking 
> in some places), but if this performance hit can't be mitigated, it may be 
> unusable for me.
> 
> For reference, this is the callback function:
> 
> ``` 
> static int cb_count = 0;
> bool SimpleCallback (
> void *baton,  
> lldb::SBProcess ,
> lldb::SBThread ,
> lldb::SBBreakpointLocation ) {
>   //TODO: Eventually do more interesting things...
>   cb_count++; 
>   return false;
> }
> ```
> 
> And here is how I set it up to be called back:
> 
> ```
> lldb::SBBreakpoint bp1 = 
> debugger_data->target.BreakpointCreateByLocation(file_name, line_no);
> if (!bp1.IsValid()) std::cerr << "invalid breakpoint";
> bp1.SetCallback(SimpleCallback, 0);
> ```
> 
> -Benjamin
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[lldb-dev] Breakpoint + callback performance ... Can it be faster?

[Benjamin Dicken via lldb-dev]

I recently started using lldb to write a basic instrumentation tool for
tracking the values of variables at various code-points in a program. I've
been working with lldb for less than two weeks, so I am pretty new. Though,
I have used and written llvm passes in the past, so I'm familiar with the
clang/llvm/lldb ecosystem.

I have a very early prototype of the tool up and running, using the C++
API. The user can specify either an executable to run or an already-running
PID to attach to. The user also supplies a file+line_number at which a
breakpoint (with a callback) is placed. For testing/prototyping purposes,
the breakpoint callback just increments a counter and then immediately
returns false. Eventually, more interesting things will happen in this
callback.

I've noticed that just the action of hitting a breakpoint and invoking the
callback is very expensive. I did some instruction-count collection by
running this lldb tool on a simple test program, and placing the
breakpoint+callback at different points in the program, causing it to get
triggered different amounts of times. I used `perf stat -e instructions
...` to gather instruction exec counts for each run. After doing a little
math, it appears that I'm incurring 1.0 - 1.1 million instruction execs per
breakpoint.

This amount of slowdown is prohibitively expensive for my needs, because I
want to place callbacks in hot portions of the "inferior" program.

Is there a way to make this faster? Is it possible to create
"lighter-weight" breakpoints? I really like the lldb API (though the
documentation is lacking in some places), but if this performance hit can't
be mitigated, it may be unusable for me.

For reference, this is the callback function:

```
static int cb_count = 0;
bool SimpleCallback (
void *baton,
lldb::SBProcess ,
lldb::SBThread ,
lldb::SBBreakpointLocation ) {
  //TODO: Eventually do more interesting things...
  cb_count++;
  return false;
}
```

And here is how I set it up to be called back:

```
lldb::SBBreakpoint bp1 =
debugger_data->target.BreakpointCreateByLocation(file_name, line_no);
if (!bp1.IsValid()) std::cerr << "invalid breakpoint";
bp1.SetCallback(SimpleCallback, 0);
```

-Benjamin
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[lldb-dev] Code Coverage with GCOV

[Ravitheja Addepally via lldb-dev]

Hello,
   I was trying to get Code Coverage for LLDB with GCOV on Linux by
adding the corresponding flags in the CMake files, but I am facing problem
with lldb-server, which does not give seem to generate the gcda files. The
host side does generate the gcov files although. The lldb-server does
generate the gcov files if it is started in Standalone mode but it does not
generate the gcov files if started by LLDB. Has anyone tried this already ?
How can I get Code Coverage for lldb / lldb-server on Linux ?


Best Regards,
A Ravi Theja
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