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Type: Improvement
Resolution: Done
Priority: Minor - P4
Fix Version/s: None
Affects Version/s: 2.6.0-rc2
Component/s: Index Maintenance, Performance
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Execution Team 2019-08-26

The database is larger than RAM and I am creating an index in the foreground. The shared_ptr destructor was frequently active when I used call stack sampling. The shared_count destructor used by shared_ptr was the #3 CPU consumer per Linux perf.

I don't have much experience with modern C++. My experience with shared_ptr is mostly limited to finding that it uses too much CPU. Is the destructor getting called frequently because the copy constructor was invoked frequently?

    13.46%   mongod  mongod               [.] mongo::BSONObjIterator::next()                                                                                                                                                  
    11.56%   mongod  mongod               [.] mongo::BSONElement::size() const                                                                                                                                                            
     9.65%   mongod  mongod               [.] boost::detail::shared_count::~shared_count()                                                                                                                                              
     9.10%   mongod  mongod               [.] mongo::compareElementValues(mongo::BSONElement const&, mongo::BSONElement const&)                                                                                      
     6.40%   mongod  mongod               [.] mongo::BSONObj::woCompare(mongo::BSONObj const&, mongo::Ordering const&, bool) const                                                                      
     5.50%   mongod  libc-2.13.so         [.] __strlen_sse42

An example is:

boost::detail::shared_count::~shared_count,~shared_ptr,~ComparatorWithInterruptCheck,~STLComparator,std::pop_heap<__gnu_cxx::__normal_iterator<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj,,mongo::sorter::MergeIterator<mongo::BSONObj,,mongo::BtreeBulk::commit<mongo::BtreeData_V1>,mongo::BtreeBasedAccessMethod::commitBulk,mongo::buildAnIndex,mongo::IndexCatalog::createIndex,mongo::CmdCreateIndex::run,mongo::_execCommand,mongo::Command::execCommand,mongo::_runCommands,runCommands,mongo::newRunQuery,receivedQuery,mongo::assembleResponse,mongo::MyMessageHandler::process,mongo::PortMessageServer::handleIncomingMsg,start_thread,clone

And an example full stack trace is:

#0  boost::detail::shared_count::~shared_count (this=0x7f634a32e150, __in_chrg=<optimized out>) at src/third_party/boost/boost/smart_ptr/detail/shared_count.hpp:305
#1  0x00000000009d5c49 in ~shared_ptr (this=0x7f634a32e148, __in_chrg=<optimized out>) at src/third_party/boost/boost/smart_ptr/shared_ptr.hpp:164
#2  ~ComparatorWithInterruptCheck (this=0x7f634a32e140, __in_chrg=<optimized out>) at src/mongo/db/extsort.cpp:41
#3  ~STLComparator (this=0x7f634a32e140, __in_chrg=<optimized out>) at src/mongo/db/sorter/sorter.cpp:359
#4  std::pop_heap<__gnu_cxx::__normal_iterator<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream>*, std::vector<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream>, std::allocator<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream> > > >, mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::STLComparator>(__gnu_cxx::__normal_iterator<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream>*, std::vector<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream>, std::allocator<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream> > > >, __gnu_cxx::__normal_iterator<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream>*, std::vector<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream>, std::allocator<boost::shared_ptr<mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::Stream> > > >, mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::STLComparator) (__first=..., __last=..., __comp=...) at /mnt/gvfs/third-party/d28c90311ca14f9f0b2bb720f4e34b285513d4f4/gcc-4.6.2-glibc-2.13/libgcc/libgcc-4.6.2/b4e37bb/include/c++/4.6.2/bits/stl_heap.h:362#5  0x00000000009d6ade in mongo::sorter::MergeIterator<mongo::BSONObj, mongo::DiskLoc, mongo::{anonymous}::ComparatorWithInterruptCheck>::next(void) (this=0x36d8b00) at src/mongo/db/sorter/sorter.cpp:325#6  0x0000000000a334c3 in mongo::BtreeBulk::commit<mongo::BtreeData_V1> (this=0x1b23540, dupsToDrop=0x7f634a32e660, op=0x36ed000, mayInterrupt=true) at src/mongo/db/index/btree_based_access_method.cpp:406#7  0x0000000000a31cef in mongo::BtreeBasedAccessMethod::commitBulk (this=0x370e4e0, bulkRaw=0x1b23540, mayInterrupt=true, dupsToDrop=0x7f634a32e660) at src/mongo/db/index/btree_based_access_method.cpp:555#8  0x00000000008a31f4 in mongo::buildAnIndex (collection=0x3704680, btreeState=0x36f1700, mayInterrupt=true) at src/mongo/db/catalog/index_create.cpp:266#9  0x000000000089efef in mongo::IndexCatalog::createIndex (this=0x3704700, spec=..., mayInterrupt=true, shutdownBehavior=mongo::IndexCatalog::SHUTDOWN_CLEANUP) at src/mongo/db/catalog/index_catalog.cpp:334#10 0x00000000008cfa0f in mongo::CmdCreateIndex::run (this=<optimized out>, dbname=..., cmdObj=..., options=<optimized out>, errmsg=..., result=..., fromRepl=false) at src/mongo/db/commands/create_indexes.cpp:176#11 0x000000000095d7bf in mongo::_execCommand (c=<optimized out>, dbname=..., cmdObj=..., queryOptions=<optimized out>, errmsg=..., result=..., fromRepl=false) at src/mongo/db/dbcommands.cpp:1357#12 0x000000000095e2af in mongo::Command::execCommand (c=0x193fe60 <mongo::cmdCreateIndex>, client=..., queryOptions=0, cmdns=0x3189414 "iibench.$cmd", cmdObj=..., result=..., fromRepl=false) at src/mongo/db/dbcommands.cpp:1544#13 0x000000000095eded in mongo::_runCommands (ns=0x3189414 "iibench.$cmd", _cmdobj=..., b=..., anObjBuilder=..., fromRepl=false, queryOptions=0) at src/mongo/db/dbcommands.cpp:1648#14 0x0000000000b7e0cc in runCommands (queryOptions=<optimized out>, anObjBuilder=..., b=..., curop=..., jsobj=..., ns=0x3189414 "iibench.$cmd", fromRepl=<optimized out>) at src/mongo/db/query/new_find.cpp:107#15 mongo::newRunQuery (m=..., q=..., curop=..., result=...) at src/mongo/db/query/new_find.cpp:384#16 0x0000000000a57860 in receivedQuery (m=..., dbresponse=..., c=...) at src/mongo/db/instance.cpp:269#17 mongo::assembleResponse (m=..., dbresponse=..., remote=...) at src/mongo/db/instance.cpp:433#18 0x00000000007a7d02 in mongo::MyMessageHandler::process (this=<optimized out>, m=..., port=0x1b22f50, le=0x1b23c70) at src/mongo/db/db.cpp:204#19 0x0000000000e40dd7 in mongo::PortMessageServer::handleIncomingMsg (arg=0x36c4d20) at src/mongo/util/net/message_server_port.cpp:209#20 0x00007f6349f03f81 in start_thread (arg=0x7f634a331700) at pthread_create.c:301#21 0x00007f634928185d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:115

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