Red Hat Bugzilla – Bug 97586
ps reports incorrect size for multithreaded programs
Last modified: 2007-04-18 12:54:53 EDT
From Bugzilla Helper: User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030529 Description of problem: [jks@xx ~] ps ax -w -w -o pid,rss,size,args --sort size PID RSS SZ COMMAND <snip> 30993 66024 132424 ./mozilla-bin 8093 9212 178764 java <snip> 8277 10832 182112 java <snip> 8208 16840 183296 java <snip> 671 6140 184484 rmiregistry 8335 11392 378684 java <snip> 21648 31948 661340 java <snip> 8151 17660 907776 java <snip> [jks@xx ~] free total used free shared buffers cached Mem: 514592 507468 7124 0 15996 142692 -/+ buffers/cache: 348780 165812 Swap: 1305568 312140 993428 If the total memory used on the system is about 820 megs, then obviously one java app can't be taking up more than 900 megs by itself. Version-Release number of selected component (if applicable): procps-2.0.11-6 How reproducible: Always Steps to Reproduce: 1. Run a program with multiple threads (like mozilla) 2. compare ps size with rss. If swap space isn't being used then size should be higher than rss.
This is almost certainly the virtual memory for the thread stacks showing up. Unless apps change the thread stack size they get an 8 meg stack by default. However, the stack is mapped in a way that the memory isn't actually allocated until it is used. Looking at 'pmap <mozilla-pid>' on my machine i see this: 42134000 (4 KB) ---p (00:00 0) 42135000 (8192 KB) rwxp (00:00 0) 42935000 (4 KB) ---p (00:00 0) 42936000 (8192 KB) rwxp (00:00 0) 43136000 (4 KB) ---p (00:00 0) 43137000 (8192 KB) rwxp (00:00 0) 43937000 (4 KB) ---p (00:00 0) 43938000 (8192 KB) rwxp (00:00 0) 44138000 (4 KB) ---p (00:00 0) 44139000 (8192 KB) rwxp (00:00 0) 44939000 (4 KB) ---p (00:00 0) 4493a000 (8192 KB) rwxp (00:00 0) Those are the thread stacks and the stack overflow protection pages.
So how is this not a bug? It is still reporting more memory used than is actually used.
No, its not. The RSS field reports how much memory is used, and the size field how much is mapped.
RSS shows how much of actual memory the process is using, excluding memory that currently resides in swap. size is supposed to be the total size, not the "size that is mapped". If you want to say that SIZE is supposed to show how much memory is mapped, then we need another option to show the real size, so this is a bug either way.
How do you define "total size" then?
size should report the memory allocated, including memory in swap. If you kill the program that is how much free memory you will gain. I really don't see how you can be confused by this.
Maybe mapped was a bad word. The threadstack memory is anonymous memory allocated to the process, and not mapped to a file, so its real allocated memory. However, its pages are initially only allocated in the page table, and actual pages are not allocated on swap/ram until the page in question is written to (copy-on-write versions of the zero page). There is no way to find out from the kernel how much of this memory is "really" allocated. However, looking at this i noticed a strange thing, it seems the threadstack memory gets displayed as data, and not swap. I wonder why VM_GROWSDOWN is not set on the thread stack mappings.
It's amazing: ps on redhat 8 doesn't work like this, top on redhat 9 doesn't work like this, but you still won't acknowledge that this is a bug.
Doesn't look like a bug to me. SZ is allocated address space, not allocated memory. Always has been. If an app has buttloads of threads, most of which have never used much of their stack allocation, the total of the SZ column in ps could indeed be lower than the total of the 'used' column in the output of free. SZ can include address space for which neither physical memory nor swap space has been committed yet.
Maybe I misstated the problem. Why is ps on redhat 9 using SZ when I passed in 'size' as the argument? redhat 8 doesn't have this problem. rh8># ps -w -w -o pid,rss,size,sz,args 478 PID RSS SIZE SZ COMMAND 478 17364 17368 238551 java <snip> rh9># ps -w -w -o pid,rss,size,sz,args 5359 PID RSS SZ SZ COMMAND 5359 14320 179740 55927 java <snip> ... so maybe the ps on rh9 got sz and size backward? Except 55927 looks a little big for what I expect my java program to be (more like 20000).
Actually I might have a rawhide version of procps on my rh8 machine, so I can't say that size worked in redhat 8.
Looking at the source code, it seems that in redhat 9 m_size is what I want, which on redhat 8 produced "-". Was there any reason in particular to rename size to m_size? On rh9 the 'size' argument calls this: /* approximation to: kB of address space that could end up in swap */ static int pr_swapable(void) { return sprintf(outbuf, "%lld", pp->vm_data + pp->vm_stack); } In redhat 8 there doesn't appear to be any argument that resembeles that.
Robert Love, the current procps maintainer has this to say: #97586 is not a bug So. For the n:th time i will close this bug. reopening it again will not help you. If you have ideas about how procps should report size, bring it up at procps-list@redhat.com where the procps developers can answer you.