MSDN Magazine - June 2008 - (Page 21)

Figure 6 VM Space Fragmentation 0:000> !address 00000000 : 00000000 - 00010000 Type 00000000 Protect 00000001 PAGE_NOACCESS State 00010000 MEM_FREE Usage RegionUsageFree 00010000 : 00010000 - 00002000 Type 00020000 MEM_PRIVATE Protect 00000004 PAGE_READWRITE State 00001000 MEM_COMMIT Usage RegionUsageEnvironmentBlock 00012000 : 00012000 - 0000e000 Type 00000000 Protect 00000001 PAGE_NOACCESS State 00010000 MEM_FREE Usage RegionUsageFree [omitted] Usage SUMMARY TotSize ( KB) Pct(Tots) Pct(Busy) Usage 701000 ( 7172) : 00.34% 20.69% : RegionUsageIsVAD 7de15000 ( 2062420) : 98.35% 00.00% : RegionUsageFree 1452000 ( 20808) : 00.99% 60.02% : RegionUsageImage 300000 ( 3072) : 00.15% 08.86% : RegionUsageStack 3000 ( 12) : 00.00% 381000 ( 3588) : 00.17% 0( 0) : 00.00% 1000 ( 4) : 00.00% 1000 ( 4) : 00.00% RegionUsageProcessParametrs 2000 ( 8) : 00.00% RegionUsageEnvironmentBlock Tot: 7fff0000 (2097088 KB) 00.03% 10.35% 00.00% 00.01% 00.01% 00.02% : : : : : : RegionUsageTeb RegionUsageHeap RegionUsagePageHeap RegionUsagePeb Busy: 021db000 (34668 KB) Type SUMMARY TotSize ( KB) Pct(Tots) Usage 7de15000 ( 2062420) : 98.35% : 1452000 ( 20808) : 00.99% : MEM_IMAGE 69f000 ( 6780) : 00.32% : MEM_MAPPED 6ea000 ( 7080) : 00.34% : MEM_PRIVATE State SUMMARY TotSize ( KB) Pct(Tots) Usage 1a58000 ( 26976) : 01.29% : MEM_COMMIT 7de15000 ( 2062420) : 98.35% : MEM_FREE 783000 ( 7692) : 00.37% : MEM_RESERVE Largest free region: Base 01432000 - Size 707ee000 (1843128 KB) chunks from the OS that are larger than 8MB, I can set a breakpoint like this: bp kernel32!virtualalloc "j (dwo(@esp+8)>800000) 'kb';'g'" ways pin it. Now take your newfound LOH knowledge and go take control of the heap. MAONI STEPHENS is Senior Developer working on the garbage collector in This breaks into the debugger and shows me the callstack if VirtualAlloc is called with the allocation size greater than 8MB (0x800000) and doesn’t break into the bugger otherwise. In CLR 2.0 we added a feature called VM Hoarding that may be applicable if you are in a situation where segments (including those for both the large and small object heaps) are frequently acquired and released. To specify VM Hoarding, you specify a startup flag called STARTUP_HOARD_GC_VM via the hosting API (see go.microsoft.com/fwlink/?LinkId=116471). When you specify this, instead of releasing empty segments back to the OS, the memory on these segments is simply decommitted and put on a standby list. The segments on the standby list will be used later to satisfy new segment requests. So next time I need a new segment, I’ll use one from this standby list if I can find one that’s big enough. Note that this isn’t done for the segments that are too large. This feature is also useful for applications that want to hold onto the segments that they’ve already acquired, like some server apps that seek to avoid fragmentation of the VM space as much as they can so that they don’t cause out-of-memory errors. And they can do this since they are usually the dominating apps on the machine. I strongly recommend you carefully test your application when you use this feature and make sure it has a fairly stable memory usage. Large objects are expensive. The allocation cost is high because the CLR needs to clear the memory for a newly allocated large object to satisfy the CLR guarantee that memory for all newly allocated objects is cleared. The LOH is collected with the rest of the heap, so carefully analyze how that impacts your app’s performance. The recommendation is to reuse large objects if possible to avoid fragmentation on the managed heap and the VM space. Finally, as of right now, the LOH is not compacted as a part of collection, but that is an implementation detail that should not be relied on. So to make sure something is not moved by the GC, almsdnmagazine.com the CLR team at Microsoft. Before joining CLR, Maoni spent several years in the Operating System group at Microsoft. June 2008 21 http://go.microsoft.com/fwlink/?LinkId=116471 http://jobs.blackbaud.com http://msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - June 2008

MSDN Magazine - June 2008 Contents Toolbox CLR Inside Out Cutting Edge Patterns In Practice SAAS Concurrency Robotics Form Filler GUI Library Service Station Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - June 2008

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