llvm dev - Nov 2013 - [LLVMdev] Debug info: type uniquing for C++ and the status on building clang with "-flto -g"

On Tue, Nov 12, 2013 at 1:01 PM, David Blaikie <dblaikie at gmail.com>
wrote:
> Hi Manman,
>
> Thanks for sending this summary and progress plans - it's great to see
the
> impact your changes have had and ideas for future direction.
>
> Type uniquing for C++ is in. Some data for Xalan with -flto -g:
>> 9.9MB raw dwarf size, peak memory usage at 2.8GB
>> The raw dwarf size was 58MB, memory usage was 7GB back in May, 2013.
>> Other efforts at size reduction helped, and type uniquing improved on
top
>> of those.
>>
>> Data on building clang with "-flto -g" after type uniquing:
>>   3.4GB MDNodes after parsing all bc files, 7GB MDNodes after linking
all
>> bc files
>>
>
> What's the change between parsing and linking?
>
Parsing means reading in all bc files to source modules. Linking means
linking in the source modules to the destination module.
Extra MDNodes can be generated for the destination module.

>
>>    4.6GB DIEs
>>
>
> It seems like the DIEs are a substantial (more than the pre-linked, but
> post-parsed BC files) part of the footprint. I think it might be important
> to do the CU-at-a-time work sooner rather than later as I'm concerned
about
> the design impact it will have on existing and future work (it's
already
> going to substantially change the cross-CU-DIE references, potentially
> changing the cost/benefit of that feature since we cannot inject DIEs from
> later CUs into prior ones).
>
>
>>   4G MCContext
>>
>
> What's the data in the MCContext that's relevant to debug info?
>
One data point on "Xalan":
without -g, MCContext allocates 45MB,
with -g, MCContext allocates 286MB.

>
>
>>   --> The memory usage is still too big.
>>
>
>  Do we have an idea of what size is "small enough"? It would be
useful to
> have a goal.
>
>
>>  So how to reduce the memory footprint at MDNode level:
>>   1> Combine integers into MDString and further combining MDStrings
(see
>> PR17891)
>>        A partial implementation on the important debug info nodes can
>> reduce the MDNodes from 7GB to 5.7GB
>>
>
> I think this'll be an interesting, and potentially valuable, change
even
> in non-LTO cases, but not necessarily where I would start just now.
>
>
>>   2> Release MDNodes that are only used by source modules (I will
send
>> out a proposal)
>>          An estimation based on partial implementation: this will
reduce
>> MDNodes from 5.7GB to 3.9GB
>>
>
> I'll keep an eye out for your proposal, as I can't quite picture
what
> you've got in mind from this brief description.
>
Yes, I plan to send out the proposal today or tomorrow.

Manman

>
>
> - David
>
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On Tue, Nov 12, 2013 at 2:08 PM, Manman Ren <manman.ren at gmail.com>
wrote:
>
>
>
> On Tue, Nov 12, 2013 at 1:01 PM, David Blaikie <dblaikie at
gmail.com> wrote:
>
>> Hi Manman,
>>
>> Thanks for sending this summary and progress plans - it's great to
see
>> the impact your changes have had and ideas for future direction.
>>
>> Type uniquing for C++ is in. Some data for Xalan with -flto -g:
>>> 9.9MB raw dwarf size, peak memory usage at 2.8GB
>>> The raw dwarf size was 58MB, memory usage was 7GB back in May,
2013.
>>> Other efforts at size reduction helped, and type uniquing improved
on
>>> top of those.
>>>
>>> Data on building clang with "-flto -g" after type
uniquing:
>>>   3.4GB MDNodes after parsing all bc files, 7GB MDNodes after
linking
>>> all bc files
>>>
>>
>> What's the change between parsing and linking?
>>
> Parsing means reading in all bc files to source modules. Linking means
> linking in the source modules to the destination module.
> Extra MDNodes can be generated for the destination module.
>
OK, that's perhaps strange - do you have any ideas about what MDNodes we
create when linking modules together? If anyhting I would expect a
reduction in size as MDNodes are deduplicated across multiple modules. Are
you measuring this after the original modules have been unloaded? Are we
not unloading those modules once we've created the merged module?

>
>
>>
>>>    4.6GB DIEs
>>>
>>
>> It seems like the DIEs are a substantial (more than the pre-linked, but
>> post-parsed BC files) part of the footprint. I think it might be
important
>> to do the CU-at-a-time work sooner rather than later as I'm
concerned about
>> the design impact it will have on existing and future work (it's
already
>> going to substantially change the cross-CU-DIE references, potentially
>> changing the cost/benefit of that feature since we cannot inject DIEs
from
>> later CUs into prior ones).
>>
>>
>>>   4G MCContext
>>>
>>
>> What's the data in the MCContext that's relevant to debug info?
>>
>
> One data point on "Xalan":
> without -g, MCContext allocates 45MB,
> with -g, MCContext allocates 286MB.
>
OK, might be useful to understand which parts of that - maybe the Values
(ints, strings, etc) themselves are being attributed to the MCContext
rather than the MDNode sizes you were reporting above? Not really sure.
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On Nov 12, 2013, at 2:08 PM, Manman Ren <manman.ren at gmail.com> wrote:
> 
> 
> 
> On Tue, Nov 12, 2013 at 1:01 PM, David Blaikie <dblaikie at
gmail.com> wrote:
> Hi Manman,
> 
> Thanks for sending this summary and progress plans - it's great to see
the impact your changes have had and ideas for future direction.
> 
> Type uniquing for C++ is in. Some data for Xalan with -flto -g:
> 9.9MB raw dwarf size, peak memory usage at 2.8GB
> The raw dwarf size was 58MB, memory usage was 7GB back in May, 2013.
> Other efforts at size reduction helped, and type uniquing improved on top
of those.
> 
> Data on building clang with "-flto -g" after type uniquing:
>   3.4GB MDNodes after parsing all bc files, 7GB MDNodes after linking all
bc files
> 
> What's the change between parsing and linking?
> Parsing means reading in all bc files to source modules. Linking means
linking in the source modules to the destination module.
> Extra MDNodes can be generated for the destination module.
Can you give an example?  I'm just curious.
> 
>  
>   4.6GB DIEs
> 
> It seems like the DIEs are a substantial (more than the pre-linked, but
post-parsed BC files) part of the footprint. I think it might be important to do
the CU-at-a-time work sooner rather than later as I'm concerned about the
design impact it will have on existing and future work (it's already going
to substantially change the cross-CU-DIE references, potentially changing the
cost/benefit of that feature since we cannot inject DIEs from later CUs into
prior ones).
>  
>   4G MCContext
> 
> What's the data in the MCContext that's relevant to debug info?
> 
> One data point on "Xalan":
> without -g, MCContext allocates 45MB,
> with -g, MCContext allocates 286MB.
So _something_ big is in the MCContext….  what is it?
>  
>  
>   --> The memory usage is still too big.
> 
> Do we have an idea of what size is "small enough"? It would be
useful to have a goal.
>  
> So how to reduce the memory footprint at MDNode level:
>   1> Combine integers into MDString and further combining MDStrings (see
PR17891)
>        A partial implementation on the important debug info nodes can
reduce the MDNodes from 7GB to 5.7GB
> 
> I think this'll be an interesting, and potentially valuable, change
even in non-LTO cases, but not necessarily where I would start just now.
>  
>   2> Release MDNodes that are only used by source modules (I will send
out a proposal)
>         An estimation based on partial implementation: this will reduce
MDNodes from 5.7GB to 3.9GB
> 
> I'll keep an eye out for your proposal, as I can't quite picture
what you've got in mind from this brief description.
> 
> Yes, I plan to send out the proposal today or tomorrow.
> 
> Manman
>  
> 
> 
> - David
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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On Tue, Nov 12, 2013 at 2:20 PM, David Blaikie <dblaikie at gmail.com>
wrote:
>
>
>
> On Tue, Nov 12, 2013 at 2:08 PM, Manman Ren <manman.ren at gmail.com>
wrote:
>
>>
>>
>>
>> On Tue, Nov 12, 2013 at 1:01 PM, David Blaikie <dblaikie at
gmail.com>wrote:
>>
>>> Hi Manman,
>>>
>>> Thanks for sending this summary and progress plans - it's great
to see
>>> the impact your changes have had and ideas for future direction.
>>>
>>> Type uniquing for C++ is in. Some data for Xalan with -flto -g:
>>>> 9.9MB raw dwarf size, peak memory usage at 2.8GB
>>>> The raw dwarf size was 58MB, memory usage was 7GB back in May,
2013.
>>>> Other efforts at size reduction helped, and type uniquing
improved on
>>>> top of those.
>>>>
>>>> Data on building clang with "-flto -g" after type
uniquing:
>>>>   3.4GB MDNodes after parsing all bc files, 7GB MDNodes after
linking
>>>> all bc files
>>>>
>>>
>>> What's the change between parsing and linking?
>>>
>> Parsing means reading in all bc files to source modules. Linking means
>> linking in the source modules to the destination module.
>> Extra MDNodes can be generated for the destination module.
>>
>
> OK, that's perhaps strange - do you have any ideas about what MDNodes
we
> create when linking modules together? If anyhting I would expect a
> reduction in size as MDNodes are deduplicated across multiple modules. Are
> you measuring this after the original modules have been unloaded? Are we
> not unloading those modules once we've created the merged module?
>
We don't unload the source modules. Even when we unload the source modules,
the MDNodes belong to the Context, they are shared among the modules.
My proposal is going to suggest an interface to delete the source modules
and remove the MDNodes used only by the source modules from the Context.

There are a few cases where we generate MDNodes when linking modules:
1> when a MDNode points to a value that is different from the source module
such as Function*.
2> when we have a cycle in the MDNode graph, all nodes in the cycle will be
created for the destination module.

When we load in the source modules, the types are already de-duplicated
(i.e multiple source modules will share the same type if possible).

>
>>
>>
>>>
>>>>    4.6GB DIEs
>>>>
>>>
>>> It seems like the DIEs are a substantial (more than the pre-linked,
but
>>> post-parsed BC files) part of the footprint. I think it might be
important
>>> to do the CU-at-a-time work sooner rather than later as I'm
concerned about
>>> the design impact it will have on existing and future work
(it's already
>>> going to substantially change the cross-CU-DIE references,
potentially
>>> changing the cost/benefit of that feature since we cannot inject
DIEs from
>>> later CUs into prior ones).
>>>
>>>
>>>>   4G MCContext
>>>>
>>>
>>> What's the data in the MCContext that's relevant to debug
info?
>>>
>>
>> One data point on "Xalan":
>> without -g, MCContext allocates 45MB,
>> with -g, MCContext allocates 286MB.
>>
>
> OK, might be useful to understand which parts of that - maybe the Values
> (ints, strings, etc) themselves are being attributed to the MCContext
> rather than the MDNode sizes you were reporting above? Not really sure.
>
Same here. I will look into that when I have time. Or somebody else already
has the answer?

Manman

>
>
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