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How can a C++ application use more Heap than RSS Memory?

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1

0

I have a C++ executable for a multithreads application which involves the opening of UDP ports.

I'm measuring the memory usage of this application using different tools.

From my understanding of Linux memory usage, RSS = Heap + Stack + Shared + CodeSize.

However, now I think that the equation is wrong, because my application has:

• Peak Heap usage: 40Mb


• Peak RSS: 30Mb

I measured the numbers with different tools (valgrind, heaptrack, top..) so I can say that they are correct.

How can it be possible?
What are the causes and the consequences of this situation?

Thanks

c++ memory heap

share|improve this question

edited Mar 27 at 15:31

user4581301

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asked Mar 27 at 15:29

alsoraalsora

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• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If I well understand RSS, this is the amout of RAM used at a certain point of time. So, this only means that all the address space of your program is not loaded all in RAM. There's surely some part of it on the swap
  
  – Gojita
  Mar 27 at 15:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Wiki page for RSS explains this quite well. RSS is how much of your memory is actually in RAM. There are many reasons why memory allocated by your program may not be in RAM yet or anymore.
  
  – user4581301
  Mar 27 at 15:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It is all about the paging i.e. some pages are on swapping partition when peek period, since they are more likely not used by the code (cold zone) and kernel understanding this and moving it from RAM to HDD.
  
  – Victor Gubin
  Mar 27 at 15:38
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @SergeiKurenkov The SWAP column from top always shows 0 during my application execution.
  
  – alsora
  Mar 27 at 15:43
  
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Memory allocated but not yet used will not yet have main memory assigned. If you allocate a huge array, you may not see an increase in RSS. If you access the first and last element in the array, you may only see the pages that contain the first and last elements, and nothing between, in RSS.
  
  – user4581301
  Mar 27 at 15:45
  

  
  
  
  

 | 
show 2 more comments

1

0

I have a C++ executable for a multithreads application which involves the opening of UDP ports.

I'm measuring the memory usage of this application using different tools.

From my understanding of Linux memory usage, RSS = Heap + Stack + Shared + CodeSize.

However, now I think that the equation is wrong, because my application has:

• Peak Heap usage: 40Mb


• Peak RSS: 30Mb

I measured the numbers with different tools (valgrind, heaptrack, top..) so I can say that they are correct.

How can it be possible?
What are the causes and the consequences of this situation?

Thanks

c++ memory heap

share|improve this question

edited Mar 27 at 15:31

user4581301

22.9k55 gold badges2222 silver badges3737 bronze badges

asked Mar 27 at 15:29

alsoraalsora

36622 silver badges1212 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If I well understand RSS, this is the amout of RAM used at a certain point of time. So, this only means that all the address space of your program is not loaded all in RAM. There's surely some part of it on the swap
  
  – Gojita
  Mar 27 at 15:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Wiki page for RSS explains this quite well. RSS is how much of your memory is actually in RAM. There are many reasons why memory allocated by your program may not be in RAM yet or anymore.
  
  – user4581301
  Mar 27 at 15:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It is all about the paging i.e. some pages are on swapping partition when peek period, since they are more likely not used by the code (cold zone) and kernel understanding this and moving it from RAM to HDD.
  
  – Victor Gubin
  Mar 27 at 15:38
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @SergeiKurenkov The SWAP column from top always shows 0 during my application execution.
  
  – alsora
  Mar 27 at 15:43
  
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Memory allocated but not yet used will not yet have main memory assigned. If you allocate a huge array, you may not see an increase in RSS. If you access the first and last element in the array, you may only see the pages that contain the first and last elements, and nothing between, in RSS.
  
  – user4581301
  Mar 27 at 15:45
  

  
  
  
  

 | 
show 2 more comments

I have a C++ executable for a multithreads application which involves the opening of UDP ports.

I'm measuring the memory usage of this application using different tools.

From my understanding of Linux memory usage, RSS = Heap + Stack + Shared + CodeSize.

However, now I think that the equation is wrong, because my application has:

• Peak Heap usage: 40Mb


• Peak RSS: 30Mb

I measured the numbers with different tools (valgrind, heaptrack, top..) so I can say that they are correct.

How can it be possible?
What are the causes and the consequences of this situation?

Thanks

c++ memory heap

share|improve this question

edited Mar 27 at 15:31

user4581301

22.9k55 gold badges2222 silver badges3737 bronze badges

asked Mar 27 at 15:29

alsoraalsora

36622 silver badges1212 bronze badges

share|improve this question

edited Mar 27 at 15:31

user4581301

22.9k55 gold badges2222 silver badges3737 bronze badges

asked Mar 27 at 15:29

alsoraalsora

36622 silver badges1212 bronze badges

share|improve this question

edited Mar 27 at 15:31

user4581301

22.9k55 gold badges2222 silver badges3737 bronze badges

asked Mar 27 at 15:29

alsoraalsora

36622 silver badges1212 bronze badges

edited Mar 27 at 15:31

user4581301

22.9k55 gold badges2222 silver badges3737 bronze badges

edited Mar 27 at 15:31

user4581301

22.9k55 gold badges2222 silver badges3737 bronze badges

asked Mar 27 at 15:29

alsoraalsora

36622 silver badges1212 bronze badges

asked Mar 27 at 15:29

alsoraalsora

36622 silver badges1212 bronze badges

2 Answers
2

active

oldest

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0

As @user4581301 pointed out in the comments, there was a problem in my idea of RSS.

HEAP = DynamicMemoryAllocated

RSS = DynamicMemoryInitialized + Stack + Shared + CodeSize

So it is not true that all the heap size is contained in the RSS, but only the part of it which has been initialized.

If you allocate space for a 10Mb array, this is HEAP.
If you start initializing it, then the initialized parts will be counted as RSS.

share|improve this answer

answered Apr 10 at 10:19

alsoraalsora

36622 silver badges1212 bronze badges

add a comment | 

0

RSS is the Resident Set Size.

Not all the heap is necessarily resident - it may be swapped out, for example.

However, the Virtual memory usage will include all of the heap (both resident and non-resident portions).

share|improve this answer

answered Apr 10 at 10:50

Toby SpeightToby Speight

18.4k1313 gold badges4949 silver badges7070 bronze badges

add a comment | 

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0

As @user4581301 pointed out in the comments, there was a problem in my idea of RSS.

HEAP = DynamicMemoryAllocated

RSS = DynamicMemoryInitialized + Stack + Shared + CodeSize

So it is not true that all the heap size is contained in the RSS, but only the part of it which has been initialized.

If you allocate space for a 10Mb array, this is HEAP.
If you start initializing it, then the initialized parts will be counted as RSS.

share|improve this answer

answered Apr 10 at 10:19

alsoraalsora

36622 silver badges1212 bronze badges

add a comment | 

0

As @user4581301 pointed out in the comments, there was a problem in my idea of RSS.

HEAP = DynamicMemoryAllocated

RSS = DynamicMemoryInitialized + Stack + Shared + CodeSize

So it is not true that all the heap size is contained in the RSS, but only the part of it which has been initialized.

If you allocate space for a 10Mb array, this is HEAP.
If you start initializing it, then the initialized parts will be counted as RSS.

share|improve this answer

answered Apr 10 at 10:19

alsoraalsora

36622 silver badges1212 bronze badges

add a comment | 

As @user4581301 pointed out in the comments, there was a problem in my idea of RSS.

HEAP = DynamicMemoryAllocated

RSS = DynamicMemoryInitialized + Stack + Shared + CodeSize

So it is not true that all the heap size is contained in the RSS, but only the part of it which has been initialized.

If you allocate space for a 10Mb array, this is HEAP.
If you start initializing it, then the initialized parts will be counted as RSS.

share|improve this answer

answered Apr 10 at 10:19

alsoraalsora

36622 silver badges1212 bronze badges

share|improve this answer

answered Apr 10 at 10:19

alsoraalsora

36622 silver badges1212 bronze badges

answered Apr 10 at 10:19

alsoraalsora

36622 silver badges1212 bronze badges

answered Apr 10 at 10:19

alsoraalsora

36622 silver badges1212 bronze badges

0

RSS is the Resident Set Size.

Not all the heap is necessarily resident - it may be swapped out, for example.

However, the Virtual memory usage will include all of the heap (both resident and non-resident portions).

share|improve this answer

answered Apr 10 at 10:50

Toby SpeightToby Speight

18.4k1313 gold badges4949 silver badges7070 bronze badges

add a comment | 

0

RSS is the Resident Set Size.

Not all the heap is necessarily resident - it may be swapped out, for example.

However, the Virtual memory usage will include all of the heap (both resident and non-resident portions).

share|improve this answer

answered Apr 10 at 10:50

Toby SpeightToby Speight

18.4k1313 gold badges4949 silver badges7070 bronze badges

add a comment | 

RSS is the Resident Set Size.

Not all the heap is necessarily resident - it may be swapped out, for example.

However, the Virtual memory usage will include all of the heap (both resident and non-resident portions).

share|improve this answer

answered Apr 10 at 10:50

Toby SpeightToby Speight

18.4k1313 gold badges4949 silver badges7070 bronze badges

share|improve this answer

answered Apr 10 at 10:50

Toby SpeightToby Speight

18.4k1313 gold badges4949 silver badges7070 bronze badges

answered Apr 10 at 10:50

Toby SpeightToby Speight

18.4k1313 gold badges4949 silver badges7070 bronze badges

answered Apr 10 at 10:50

Toby SpeightToby Speight

18.4k1313 gold badges4949 silver badges7070 bronze badges