WCL405: Mysteries of Windows Memory Management Revealed

WCL405: Mysteries of Windows Memory Management Revealed

WCL405 Mysteries of Windows Memory Management Revealed Mark Russinovich Technical Fellow Windows Azure (created jointly with Dave Solomon) Goals Deep dive on: Process virtual and physical memory usage Operating system virtual and physical memory usage

Crisply define memory-related terminology Highlight tools that reveal memory usage Describe dark spots in memory analysis counters and tools Agenda Virtual Memory Address Space Usage Process Commit System Commit Physical Memory

Working Sets Paging Lists Hard to Track Memory Tools Well Use Task Manager Sysinternals Process Explorer Sysinternals Vmmap Process virtual and physical memory usage Sysinternals Rammap System physical memory usage

Sysinternals Testlimit Test program to leak different kinds of memory Sysinternals tools are free at www.sysinternals.com Virtual Memory Memory Management Fundamentals Windows has demand-paged memory management Processes demand memory as needed There is no swapping A page is 4 KB (8 KB on Itanium)

Allocations must align on 64 KB boundaries Large pages are available for improved TLB usage x86: 4 MB X64 and x86 PAE: 2 MB Itanium: 16 MB There is NO (will, almost no) connection between virtual memory and physical memory 32-bit x86 Address Space 32-bits = 2^32 = 4 GB /3GB and /USERVA can extend process address up to 3 GB

Process must be marked large address space aware to use memory above 2 GB Default 2 GB Per-Process Space 2 GB System Space 3 GB user space

3 GB Per-Process Space 1 GB System Space 64-bit Address Spaces 64-bits = 2^64 = 17,179,869,184 GB x64 today supports 48 bits virtual = 262,144 GB = 256 TB IA-64 today support 50 bits virtual = 1,048,576 GB = 1024 TB 64-bit Windows supports 44 bits = 16,384 GB

= 16 TB x64 8 TB Per-Process Space 8 TB System Space 4 GB Per-Process

Space 8 TB System Space 32-bit process on x64 Virtual Address Space Components Committed: in-use Reserved: reserved for future use

Address space breakdown Private (e.g. process heap) Reserved or committed Shareable (e.g. EXE, DLL, shared memory, other memory mapped files) Reserved or committed Free (not yet defined) Why Reserve Memory? Reserved memory lets an application lazily commit contiguous memory

Used for stack and heap expansion Committed Thread Stack Guard Reserved Before Expansion Stack

Grows Down Committed Guard Reserved After Expansion Viewing Address Space Breakdown Task Manager only lets you see private bytes Before Vista: column called VM Size Vista and later: column called Commit Size

Process Explorer shows both virtual size and private bytes Add 2 columns to process list Virtual Size Private Bytes Run Testlimit twice Testlimit -r Testlimit -m Note: if on 64-bit Windows, 32-bit Testlimit can grow to 4GB Understanding Process Address Space Usage

Most virtual memory problems are due to a process leaking private committed memory Heap, GC heap, language heaps (CRT) Private Bytes only tells part of the story Doesnt account for shareable memory thats not shared (e.g. DLLs loaded only by this process) Fragmentation can be an issue Fragmented Address space can effectively be exhausted prematurely Address Basic performance counters dont provide enough Space

information to troubleshoot Viewing Processes with VMMap VMMap shows detailed breakdown of process address space: Private process memory Copy-on-write Private (VirtualAlloc) Heap and GC Heap Stack Shareable process memory Image - executables

Shareable shareable memory Mapped File memory mapped files Page table page table pages Unusable gap between allocation and next allocation boundary Note that shareable types can have private commitment Read/write pages in shared memory Copy-on-write pages Viewing Fragmentation Fragmentation is visible by selecting Options->Show Free Regions, selecting the Free type, and sorting by size

Largest free block is largest allocation possible Clickable fragmentation map in View->Fragmentation View Run testlimit -t on 64-bit Windows Threads need 256 KB 64-bit stack and 1 MB 32-bit stack File Mappings File mapping enables an application to read and write file data through memory operations File mappings are used for Image (.EXE and .DLL) loading: Image in VMMap Data files access (e.g. NLS files): Mapped File in VMMap Pagefile-backed shared memory: Shareable in VMMap

Entire file doesnt have to be mapped Allows for windows into the file Database.db Address Space Tracing File Mapping with Process Monitor Procmon can trace image loader activity VMMap Differencing Press F5 to refresh the view VMMap keeps all snapshots

Use the timeline to select snapshots to compare Tracing with VMMap You can launch a process with profiling Detours tracks virtual and heap activity The System Commit Limit System committed virtual memory must be backed either by physical memory or stored in the paging file Sum of (most of) physical memory and current paging files Allocations charged against the system commit limit: Process private bytes

Pagefile-backed shared memory Copy-on-write pages Read/write file pages System paged and nonpaged code and data When limit is reached, virtual memory allocations fail Processes may crash (or corrupt data) Changing the System Commit Limit You can increase the system commit limit by adding RAM or increasing the pagefile size The system commit limit can grow if paging files configured to expand

So the system commit limit might be the current limit, not the maximum Default configuration (System Managed): Minimum: 1.5x RAM if RAM < 1 GB; RAM otherwise Maximum: 3x RAM or 4 GB, whichever is larger Maximum system commit limit should be based on system commit peak for extreme workload Viewing System Commit Usage Performance Counters: Committed Bytes Commit Limit

Task Manager XP: commit charge labeled PF Usage Vista: commit charge labeled Page File Win7: commit charge labeled Commit Vista and Win7 show commit limit after slash Viewing the System Commit Limit Process Explorer shows commit charge (with history), commit limit, and commit peak No built-in tool shows peak any more Exhausting the System Commit Limit

On 32-bit system, run Testlimit m multiple times until system commit limit exhausted On 64-bits, Testlimit64 m will exhaust the system commit limit before its address space: Sizing the Paging File If you enough RAM to support your commit needs, why even have one? System can page out unused, modified private pages vs keeping them in RAM More RAM available for useful stuff Many recommendations use a formula based on RAM (1.5x, 2x, etc.) Actually, the more RAM, the smaller the paging file needed Should be based on workload usage of committed virtual memory

Look at commit peak after workload has run Pre-Vista: Task Manager Vista+: Process Explorer Apply a formula to that to give buffer (1.5x or 2x) Make sure its big enough to hold a kernel crash dump Physical Memory Working Set List newer pages older pages

Working Set All the physical pages owned by a process E.g. the pages the process can reference without incurring a page fault A process always starts with an empty working set It then incurs page faults when referencing a page that isnt in its working set Hard fault: resolved from file on disk (paging file, mapped file) Soft fault: resolved from memory Working Set

Each process has a default working set minimum and maximum Can change with SetProcessWorkingSet Working set minimum controls maximum number of locked pages (VirtualLock) Minimum is also reserved from RAM as a guarantee to the process Working set maximum is ignored If theres ample memory, process working set represents all the memory it has referenced (but not freed) If memory is tight, working sets get trimmed

Working Set Replacement To standby or modified page list Working Set When memory manager decides the process is large enough, it give up pages to make room for new pages Local page replacement policy Means that a single process cannot take over all of physical memory unless other processes arent using it Page replacement algorithm is least recently accessed (pages are aged when available memory is low)

Working Set Breakdown Consists of 2 types of pages: Shareable (of which some may be shared) Private Four performance counters available: Working Set Shareable Working Set Shared (subset of shareable that are currently shared) Working Set Private Working Set Size (total of WS Shareable+Private) Note: adding this up for each process overcounts shared pages

Caveats: Working set does not include trimmed memory that is still cached Shareable working set should be viewed as private if its not shared Viewing Working Set with Task Manager Displays private working set size Calls it Memory (Private Working Set) Viewing Working Set with Process Explorer Process Explorer shows all the performance counters Virtual Bytes

Private Bytes WS Shareable Bytes WS Shared Bytes WS Private Bytes Run Testlimit three times: Testlimit -r 1024 -c 1 Testlimit -m 1024 -c 1 Testlimit -d 1024 -c 1 Note how working set numbers dont at all represent the process virtual memory usage

Viewing the Working Set with VMMap Vmmap shows working set size of each component of address space Also shows locked pages Copy-on-write pages will show up as Private WS in shareable regions How Copy-On-Write Works Before Orig. Data Page 1 Orig. Data

Page 2 Page 3 Process Address Space Physical memory Process Address Space

How Copy-On-Write Works After Orig. Data Page 1 Modd. Data Page 2 Page 3 Process

Address Space Copy of page 2 Physical memory Process Address Space Managing Physical Memory System keeps unassigned physical pages on one of

several lists Free page list Modified page list Standby page lists (8 as of Vista & later) Zero page list ROM page list Bad page list - pages that failed memory test at system startup Lists are implemented by entries in the PFN database Maintained as FIFO lists or queues Paging Dynamics New pages are allocated to working sets from the top of

the free or zero page list Pages released from the working set due to working set replacement go to the bottom of: The modified page list (if they were modified while in the working set) The standby page list (if not modified) Decision made based on D (dirty = modified) bit in page table entry Association between the process and the physical page is still maintained while the page is on either of these lists Standby and Modified Page Lists Modified pages go to modified (dirty) list

Avoids writing pages back to disk too soon Unmodified pages go to standby (clean) lists They form a system-wide cache of pages likely to be needed again Pages can be faulted back into a process from the standby and modified page list These are counted as page faults, but not page reads Modified Page Writer When modified list reaches certain size, modified page writer system thread is awoken to write pages out Also triggered when memory is overcommitted (too few free

pages) Does not flush entire modified page list Two system threads One for mapped files, one for the paging file Pages move from the modified list to the standby list E.g. can still be soft faulted into a working set Free and Zero Page Lists Free Page List Used for page reads Private modified pages go here on process exit

Pages contain junk in them (e.g. not zeroed) On most busy systems, this is empty Zero Page List Used to satisfy demand zero page faults References to private pages that have not been created yet When free page list has 8 or more pages, a priority zero thread is awoken to zero them On most busy systems, this is empty too Paging Dynamics demand zero page

faults page read from disk or kernel allocations (hard page faults) Standby Page Lists Working Sets soft

page faults global valid faults working set replacement modified page writer Modified

Page List Private pages at process exit Free Page List zero page thread

Zero Page List Bad Page List Viewing the Paging Lists with Task Manager XP/2003: Available = Standby + Zero + Free System Cache = Standby +

Modified + System Working Set Vista/Server 2008: Replaced Available with Free Free + Zero list System Cache relabeled Cached Windows 7/Server 2008 R2 Available put back Viewing the Paging Lists with Process Explorer Process Explorer shows each paging list

Click View->System Information Total Process Private Memory Usage Working Set size does not include: Private memory on standby or modified lists Page tables Rammap shows this on Processes tab Viewing Memory Usage with Rammap

In addition to showing size of paging lists, shows usage breakdown: Process private Mapped file Shared memory Page tables Paged pool Nonpaged pool System PTE Session private Metafile AWE Driver locked Kernel stack

Prioritized Standby Lists In Vista & later, there are 8 prioritized standby lists Pages are removed from lowest priority list first Pages removed Prioritized Standby Lists 0

1 2 Low memory priority process will keep re-using low priority pages Higher priority information remains cached 3 4 5 6 7

Pages added SuperFetch Superfetch proactively repopulates RAM with the most useful data Sets priority of pages to optimal value, based the page history and other analysis that it performs Takes into account frequency of page usage, usage of page in context of other pages in memory Adapts application launch patterns, in chunks of 8 hours (times a day) and weekend vs weekday Scenarios SuperFetch improves include Resume from hibernate and suspend

Fast user switching Performance after infrequent or low priority tasks execute Application launch Windows 7: Disabled if the OS is booted of an SSD Memory Priority Each thread has its own memory priority 5: normal 1: low This determines which standby list is used for the page (when/if it arrives on the standby list)

Thread priority comes from process memory priority Can be changed for process or individual thread SetPriorityClass or SetThreadPriority background mode Standby List Population Priority 7 come from a static set (pre-trained at Microsoft) Pre-populated at each boot Includes pages related to user input that requires fast responsiveness (right-click, desktop properties, control panel, start menu, etc.) Priority 6 are pages that SuperFetch considers important, or useful (will rarely get repurposed)

Priority 5 are standard user pages (memory priority 5) Priority 1 are low priority user pages (memory priority 1) Priority 0-4 may be Superfetch decayed, cache manager read-ahead and pagefault clustering How Much of the Standby List has Been Consumed? RAMMap shows the amount of memory repurposed off each standby list since boot: What File Data is In the Standby Lists? Viewing Cached Files with Rammap

Do You Have Enough Memory? Theres no sure-fire rule or counter to tell if you if you have enough memory The general rule: available memory remains generally low Use Perfmon to monitor available memory over time Use Process Explorer, or on Vista and later, Task Manager, to monitor physical memory usage Use Process Explorer, or Task Manager to see instantaneous value of available memory Watch in Process Monitor for excessive reads from paging file Tracing Paging with Procmon Procmon distinguishes paging I/Os in the details column Can set filter for detail contains paging

I/O to Pagefile.sys excluded by default Enable advanced output or remove exclude filter Excessive reads from paging file indicates need more RAM Hard to Track Memory Hidden Cost of Reserved Memory Memory Manager charges for page tables for reserved address space not yet committed Charged against process private bytes (and therefore system commit limit) Cannot track this down Experiment:

Testlimit64 r 100000 c 10 Testlimit64 r Reserves ~8192GB Private bytes grows to >16GB! Cost of Reserved Memory Virtual Address Space (VAD) descriptors come from nonpaged pool Example: Testlimit64 r results in 640mb of nonpaged pool usage for VADs Poolmon shows this:

Shared Memory Shared memory is backed by virtual memory Either paging file (if there is one), else physical memory However, amount created not charged to process commit limit Therefore, a shared memory VM leak is hard to track down Demo: Shared Memory Leak Testlimit s 1000 c 3 This creates a 3 GB shared memory section Note that process virtual and private bytes do not include

this value And only virtual bytes rise when process maps the section into its address space VirtualLock Locked Pages No special privilege is required to VirtualLock pages (as of Vista) Allows process to allocate non-paged memory Locked memory has a major impact on the system: Overrides memory management policies Prevents contiguous physical memory allocation Can prevent hibernation

Testlimit -l 1024 c 1 DWM locks memory: Driver Locked Pages and AWE pages VPC and Hyper-V use driver locked pages SQL Server uses AWE memory Counts against system commit, but not otherwise detectable No way to track back to owner More Information More information in Windows Internals, 5th Edition Memory Management chapter

MSDN memory management API documentation The best way to gain an understanding of memory manager behavior is to experiment and observe Come to my other sessions tomorrow! Inside Windows Azure: 10:15 Case of the Unexplained: 2:45 Book signing at the TechEd bookstore: 12:30-1:30 Track Resources Dont forget to visit the Cloud Power area within the TLC (Blue Section) to see product demos and speak with experts about the Server & Cloud Platform solutions that help drive your business forward. You can also find the latest information about our products at the following links:

Cloud Power - http://www.microsoft.com/cloud/ Private Cloud - http://www.microsoft.com/privatecloud/ Windows Server - http://www.microsoft.com/windowsserver/ Windows Azure - http://www.microsoft.com/windowsazure/ Microsoft System Center - http://www.microsoft.com/systemcenter/ Microsoft Forefront - http://www.microsoft.com/forefront/ Resources Connect. Share. Discuss. http://northamerica.msteched.com Sessions On-Demand & Community

Learnin g Microsoft Certification & Training Resources www.microsoft.com/teched www.microsoft.com/learning Resources for IT Professionals Resources for Developers

http://microsoft.com/technet http://microsoft.com/msdn Complete an evaluation on CommNet and enter to win! Scan the Tag to evaluate this session now on myTechEd

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