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Re^5: RFC: Abusing "virtual" memory (page faults vs malloc)

by BrowserUk (Pope)
on Nov 28, 2007 at 10:01 UTC ( #653503=note: print w/ replies, xml ) Need Help??


in reply to Re^4: RFC: Abusing "virtual" memory (page faults vs malloc)
in thread RFC: Abusing "virtual" memory

This displays the page fault count before and after allocating an 40960 byte chunk of memory from a heap, then freeing it and allocating it again.

#! perl -slw use strict; use Inline C => Config => LIBS => '-lpsapi.lib'; use Inline C => 'DATA', NAME => 'heap', CLEAN_AFTER_BUILD => 0; my $heap = heapCreate( 0, 0, 1024 * 1024 ); my $space = heapAlloc( $heap, 0, 4096 * 10 ); heapFree( $heap, 0, $space ); $space = heapAlloc( $heap, 0, 4096 * 10 ); print heapSize( $heap, 0, $space ); __DATA__ __C__ #include <windows.h> #include <psapi.h> U32 heapCreate( U32 flags, U32 initial, int max ) { return (U32) HeapCreate( flags, initial, max ); } U32 heapAlloc( U32 hHeap, U32 flags, U32 size ) { U32 pMem; PROCESS_MEMORY_COUNTERS pmc; pmc.cb = sizeof( PROCESS_MEMORY_COUNTERS ); GetProcessMemoryInfo( GetCurrentProcess(), &pmc, sizeof( PROCESS_MEMORY_COUNTERS ) ); printf( "pagefaults before alloc of %d bytes: %d\n", size, pmc.PageFaultCount ); pMem = (U32)HeapAlloc( (HANDLE)hHeap, flags, (SIZE_T)size ); GetProcessMemoryInfo( GetCurrentProcess(), &pmc, sizeof( PROCESS_MEMORY_COUNTERS ) ); printf( "pagefaults after alloc of %d bytes: %d\n", size, pmc.PageFaultCount ); return pMem; } U32 heapSize( U32 hHeap, U32 flags, U32 mem ) { return (U32)HeapSize( (HANDLE)hHeap, flags, (LPVOID)mem ); } U32 heapFree( U32 hHeap, U32 flags, U32 mem ) { return (U32)HeapFree( (HANDLE)hHeap, flags, (LPVOID)mem ); }

The output

c:\test>HeapMem.pl pagefaults before alloc of 40960 bytes: 952 pagefaults after alloc of 40960 bytes: 964 pagefaults before alloc of 40960 bytes: 964 pagefaults after alloc of 40960 bytes: 964 40960 c:\test>HeapMem.pl pagefaults before alloc of 40960 bytes: 953 pagefaults after alloc of 40960 bytes: 965 pagefaults before alloc of 40960 bytes: 965 pagefaults after alloc of 40960 bytes: 965 40960 c:\test>HeapMem.pl pagefaults before alloc of 40960 bytes: 953 pagefaults after alloc of 40960 bytes: 965 pagefaults before alloc of 40960 bytes: 965 pagefaults after alloc of 40960 bytes: 965 40960

Shows that the first time, allocating 10 pages of memory results in 12 page faults. The second time none.


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Comment on Re^5: RFC: Abusing "virtual" memory (page faults vs malloc)
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Re^6: RFC: Abusing "virtual" memory (page faults vs malloc)
by tye (Cardinal) on Nov 28, 2007 at 16:18 UTC

    Duh. That doesn't mean that your program trying to use lots of physical memory is going to do less page faulting. The original problem statement:

    By loading several million records into an array, the virtual-memory footprint of this application blossomed to about 44 megabytes, which caused about 75,000 page faults to occur just in loading and sorting that “memory” list. Although there was enough RAM to allow that to happen without physical I/O, Linux discarded a corresponding amount of file-buffer space ... and Linux depends a great deal on its buffers for good performance.

    So your "solution" won't prevent the problem. Granted, pre-allocating can reduce heap fragmentation and eliminate a few copyings, but it isn't going to make several million records suddenly no longer require many MBs and suddenly leaving room for most of the file buffer to remain. And a fragmented heap is likely to have less impact on the number of pages that need to be kept swapped in than it does on the total heap size. So I stand by my prediction that such games are unlikely to have a major impact in this type of situation.

    - tye        

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