[lldb-dev] LLDB performance drop from 3.9 to 4.0

Zachary Turner via lldb-dev lldb-dev at lists.llvm.org
Wed Apr 19 16:48:46 PDT 2017

The change was made to make the interface safer and allow propagation of
StringRef through other layers.  The previous code was already taking a
const char *, and so it was working under the assumption that the const
char* passed in came from a ConstString.  As such, continuing to make that
same assumption seems completely reasonable.

So perhaps you can just change the operator to compare the pointers, as was
being done before.

On Wed, Apr 19, 2017 at 4:24 PM Scott Smith <scott.smith at purestorage.com>

> It looks like it was this change:
> commit 45fb8d00309586c3f7027f66f9f8a0b56bf1cc4a
> Author: Zachary Turner <zturner at google.com>
> Date:   Thu Oct 6 21:22:44 2016 +0000
>     Convert UniqueCStringMap to use StringRef.
>     git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@283494
> 91177308-0d34-0410-b5e6-96231b3b80d8
> I'm guessing it's because the old code assumed const string, which meant
> that uniqueness comparisons could be done by simply comparing the pointer.
> Now it needs to use an actual string comparison routine.  This code:
>      bool operator<(const Entry &rhs) const { return cstring <
> rhs.cstring; }
> didn't actually change in the revision, but cstring went from 'const char
> *' to 'StringRef'.  If you know for sure that all the StringRefs come from
> ConstString, then it'd be easy enough to change the comparison, but I don't
> know how you guarantee that.
> I assume the change was made to allow proper memory cleanup when the
> symbols are discarded?
> On Thu, Apr 13, 2017 at 5:37 AM, Pavel Labath <labath at google.com> wrote:
>> Bisecting the performance regression would be extremely valuable. If you
>> want to do that, it would be very appreciated.
>> On 12 April 2017 at 20:39, Scott Smith via lldb-dev <
>> lldb-dev at lists.llvm.org> wrote:
>>> For my app I think it's largely parsing debug symbols tables for shared
>>> libraries.  My main performance improvement was to increase the parallelism
>>> of parsing that information.
>>> Funny, gdb/gold has a similar accelerator table (created when you link
>>> with -gdb-index).  I assume lldb doesn't know how to parse it.
>>> I'll work on bisecting the change.
>>> On Wed, Apr 12, 2017 at 12:26 PM, Jason Molenda <jason at molenda.com>
>>> wrote:
>>>> I don't know exactly when the 3.9 / 4.0 branches were cut, and what was
>>>> done between those two points, but in general we don't expect/want to see
>>>> performance regressions like that.  I'm more familiar with the perf
>>>> characteristics on macos, Linux is different in some important regards, so
>>>> I can only speak in general terms here.
>>>> In your example, you're measuring three things, assuming you have debug
>>>> information for MY_PROGRAM.  The first is "Do the initial read of the main
>>>> binary and its debug information".  The second is "Find all symbol names
>>>> 'main'".  The third is "Scan a newly loaded solib's symbols" (assuming you
>>>> don't have debug information from solibs from /usr/lib etc).  Technically
>>>> there's some additional stuff here -- launching the process, detecting
>>>> solibs as they're loaded, looking up the symbol context when we hit the
>>>> breakpoint, backtracing a frame or two, etc, but that stuff is rarely where
>>>> you'll see perf issues on a local debug session.
>>>> Which of these is likely to be important will depend on your
>>>> MY_PROGRAM.  If you have a 'int main(){}', it's not going to be dwarf
>>>> parsing.  If your binary only pulls in three solib's by the time it is
>>>> running, it's not going to be new module scanning. A popular place to spend
>>>> startup time is in C++ name demangling if you have a lot of solibs with C++
>>>> symbols.
>>>> On Darwin systems, we have a nonstandard accelerator table in our DWARF
>>>> emitted by clang that lldb reads.  The "apple_types", "apple_names" etc
>>>> tables.  So when we need to find a symbol named "main", for Modules that
>>>> have a SymbolFile, we can look in the accelerator table.  If that
>>>> SymbolFile has a 'main', the accelerator table gives us a reference into
>>>> the DWARF for the definition, and we can consume the DWARF lazily.  We
>>>> should never need to do a full scan over the DWARF, that's considered a
>>>> failure.
>>>> (in fact, I'm working on a branch of the llvm.org sources from
>>>> mid-October and I suspect Darwin lldb is often consuming a LOT more dwarf
>>>> than it should be when I'm debugging, I need to figure out what is causing
>>>> that, it's a big problem.)
>>>> In general, I've been wanting to add a new "perf counters"
>>>> infrastructure & testsuite to lldb, but haven't had time.  One thing I work
>>>> on a lot is debugging over a bluetooth connection; it turns out that BT is
>>>> very slow, and any extra packets we send between lldb and debugserver are
>>>> very costly.  The communication is so fast over a local host, or over a usb
>>>> cable, that it's easy for regressions to sneak in without anyone noticing.
>>>> So the original idea was hey, we can have something that counts packets for
>>>> distinct operations.  Like, this "next" command should take no more than 40
>>>> packets, that kind of thing.  And it could be expanded -- "b main should
>>>> fully parse the DWARF for only 1 symbol", or "p *this should only look up 5
>>>> types", etc.
>>>> > On Apr 12, 2017, at 11:26 AM, Scott Smith via lldb-dev <
>>>> lldb-dev at lists.llvm.org> wrote:
>>>> >
>>>> > I worked on some performance improvements for lldb 3.9, and was about
>>>> to forward port them so I can submit them for inclusion, but I realized
>>>> there has been a major performance drop from 3.9 to 4.0.  I am using the
>>>> official builds on an Ubuntu 16.04 machine with 16 cores / 32 hyperthreads.
>>>> >
>>>> > Running: time lldb-4.0 -b -o 'b main' -o 'run' MY_PROGRAM > /dev/null
>>>> >
>>>> > With 3.9, I get:
>>>> > real    0m31.782s
>>>> > user    0m50.024s
>>>> > sys    0m4.348s
>>>> >
>>>> > With 4.0, I get:
>>>> > real    0m51.652s
>>>> > user    1m19.780s
>>>> > sys    0m10.388s
>>>> >
>>>> > (with my changes + 3.9, I got real down to 4.8 seconds!  But I'm not
>>>> convinced you'll like all the changes.)
>>>> >
>>>> > Is this expected?  I get roughly the same results when compiling
>>>> llvm+lldb from source.
>>>> >
>>>> > I guess I can spend some time trying to bisect what happened.  5.0
>>>> looks to be another 8% slower.
>>>> >
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