Review Request: speed up of rxx_allocator

Sun Mar 6 12:05:03 UTC 2011

On Sat, 2011-03-05 at 18:07 +0000, Milian Wolff wrote:
> This is an automatically generated e-mail. To reply, visit:
> http://git.reviewboard.kde.org/r/100730/ 
> 
>         On February 26th, 2011, 12:14 p.m., David Nolden wrote:
>         
>                 The improvements you report from duchainify are far too huge, allociation has never played such a huge role during parsing. The problem is probably, that during the first (slow) run the duchain is built, then it is stored, and during the next run, it's only updated.
>                 
>                 Please make sure that:
>                 A) You call duchainify twice for each case, and use the second timing
>                 B) You always use the "--force-update-recursive" parameter, which should eliminate the duchain caching issues
>                 
>                 This issue also explains why you have observed the 50x difference in the number of calls to "new". From what I understand, you've changed the implementation of "new", so it shouldn't affect that number, or did I misunderstand that?
>         
>         On March 4th, 2011, 4:58 p.m., Floris Ruijter wrote:
>         
>                 I reran the test with this script:
>                 =====
>                 #!/bin/bash
>                 echo 'starting the test. first the old version.'
>                 date
>                 ~/src/kdevplatform/build/util/duchainify/duchainify --force-update-recursive ~/src/kdevelop/languages/cpp/ > /dev/null
>                 time ~/src/kdevplatform/build/util/duchainify/duchainify --force-update-recursive ~/src/kdevelop/languages/cpp/ > /dev/null
>                 
>                 echo 'building the new version.'
>                 cd ~/src/kdevelop/
>                 git apply build/0001-rewrite-rxx_allocator-and-move-includes-from-the-hea.patch
>                 cd build
>                 make install > /dev/null
>                 cd
>                 
>                 echo 'new version:'
>                 date
>                 ~/src/kdevplatform/build/util/duchainify/duchainify --force-update-recursive ~/src/kdevelop/languages/cpp/ > /dev/null
>                 time ~/src/kdevplatform/build/util/duchainify/duchainify --force-update-recursive ~/src/kdevelop/languages/cpp/ > /dev/null
>                 =====
>                 the new version was about a minut faster(6:14 vs 5:18), i also used valgrind with the iostream including file again(this time running duchainify one time solo first) and here too i could see a very significant improvement in libkdev4cppparser.so . 
>                 
>                 the patch does NOT reimplement operator new. it just changes the way rxx_allocator works. rxx_allocator tries to get blocks from a static linked list first(which is tryLocked with a QMutex) if the chain is already locked or is empty it will instead create a new block and add that to it's own linked list of blocks. on destruction of a rxx_allocator, it will lock the static chain of unallocated blocks and add it's own chain to the chain of unallocated blocks. 
>                  
>         
>         On March 4th, 2011, 5:01 p.m., Floris Ruijter wrote:
>         
>                 i wrote the comment above a week ago i somehow just forgot to "publish changes".
>                 if noone objects, i will push the changes that memorypool.h is only included in .cpps in a few days, but I'd like to hear if the changes to rxx_allocator are to be commited too. 
> just for completeness sake (if it's not too much trouble), please remove the .kdevduchain dir (at least for the session duchainify uses, afair it's always the same) before running duchainify.
> 
> I'll also try this patch now.
> 
> the change you want to push re memorypool.h is to make build time shorter, right? if you are sure it is worthwhile and safe to do so then I'm fine with merging that part. The other changes need some more reviewing/testing I'd say.
> 
> 
> 
> - Milian
> 
> 
> On February 24th, 2011, 1:47 a.m., Floris Ruijter wrote:
> 
> Review request for KDevelop.
> By Floris Ruijter.
> Updated Feb. 24, 2011, 1:47 a.m.
> 
> 
> Description 
> rxx_allocator was according to my measurements done with kcachegrind, valgrind, duchainify and iostream. The allocator had three basic defects:
> 1) all allocated memory was deallocated whilst we need a lot of rxx_allocators (1 per file i presume?), so these blocks can be reused
> 2) it cleared the memory on a per block basis, but if not all of the block is used, then that is a waste of effort
> 3) it used realloc to manage the list of blocks, this isn't too bad but could cause a move of the list which is totaly unnecessary
> 
> i solved the problems mostly by making the blocks act as linked list nodes: a next pointer + a really long char array. deallocated blocks are kept in a static linked list, whilst actual rxx_allocators have their own(personal some would say)linked list of blocks. access to the deallocated blocks list is synchronized through a static QMutex.
> 
> the access could be threadsafe by using a thread local linked list of deallocated items too, but i don't think that'd be practical, the global static list is probably more effective (eventhough it requires locking) 
> Testing 
> as mentioned i ran a file which only included iostream through duchainify which i callgrinded.
> 
>                       old:              new: 
> pool::allocate        ~450 000 000      ~7 000 000
> 
> all time spend in libkdev4cppparser:
>                       ~585 000 000      ~140 000 000
> 
> 
> the pool::allocate numbers are both the 'inclusive' numbers
> 
> looking at the data for the amount of "operator new" calls I can see that the cost per call are pretty much the same but that the old implementation called it about 50x more.
> Diffs 
>       * languages/cpp/codecompletion/item.cpp (b25d1ae)
>       * languages/cpp/cppparsejob.cpp (f4819f2)
>       * languages/cpp/parser/ast.h (0281c6b)
>       * languages/cpp/parser/control.h (0b06248)
>       * languages/cpp/parser/listnode.h (d1eda36)
>       * languages/cpp/parser/parser.cpp (281ad8d)
>       * languages/cpp/parser/rxx_allocator.h (f0159e9)
>       * languages/cpp/parser/tests/test_parser.cpp (de5f804)
> 
> View Diff
> 
> 
thank you millian, indeed the changes wrt memorypool.h are too speedup
buildtimes after changes have been made to rxx_allocator.h, it just
moves #include "memorypool.h" to cpps and then uses class pool; instead
in the header. I agree that the actual change to rxx_allocator requires
some more testing/reviewing.

@ milian's second comment: 
I did use callgrind, but it kinda sucks as you have to disable inlining
to see the actual speed of rxx_allocator. otoh, i've done several tests
and all indicate that it's faster but all tests i did with call grind
were on this:
==main.cpp==
#include <iostream>
==end==
with 
valgrind --tool=callgrind duchainify --force-update-recursive .

I did it several times and I saw a noticable speedup every time. problem
with using a larger input with valgrind is that it'l take ages to do the
testing. 

What i see is that most of the time was spent doing the block allocation
with new. The new patch tries to minimize the amount of calls to new
while not waiting. I see but one problem: the global blocklist is never
pruned, so it'll keep growing as two threads try to get a block, on of
them will get a new block and the other will get one from the chain.
eventhough the time spent holding the lock should be very small, this
will happen from time to time, so i guess there should be a system to
stop this from happening.

I'd have to look into creating QBENCHMARKS. I can do that, but it'll
take some time which I don't have a lot of this week, so if I'm going to
do it then you should probably not expect it before next weekend.

Floris