How can the Private Bytes of a process be significantly less than its effect on the system commit charge?
- by bacar
On a 64-bit Windows Server 2003, I can see using taskmgr or process explorer that the total commit charge is around 3.5GB, yet when I sum the Private Bytes consumed by each process (by running pslist -m and adding all values under the Priv column) the total comes in at 1.6GB.
I know which process seems to be causing this (sqlservr.exe) as when I kill the process, the commit charge drops dramatically. However the process in question is consuming only ~220MB of Private Bytes yet killing the process drops the commit charge by ~1.6GB.
How is this possible? How can the commit charge be so significantly greater than Private Bytes, which should represent the amount of committed memory? If some other factor contributes to the commit charge, what is that factor and how can I view its impact in process explorer?
Note: I claim that I understand the difference between reserved and committed memory already: my investigations above relate specifically to Private Bytes which includes only committed memory and excludes reserved memory. the Virtual Size of the process in this case is over 4GB, but this should be irrelevant - Virtual Size in procexp represents reserved, not committed memory, and should not contribute to the commit charge.
I'm particularly interested in generalised answers to this question: I'm assuming that if sqlservr.exe can behave in this way, that any process potentially could.
Further Investigations
I notice that pointing Sysinternals VMMap at this process reports a committed "Private Data" of 1.6GB despite Procexp's reported a Private Bytes of 220MB. This is particularly strange given that the documentation for this field in the "Windows® Sysinternals Administrator's Reference" states that:
Private Data memory is memory that is allocated by VirtualAlloc and that is not further handled by the Heap Manager or the .NET runtime, or assigned to the Stack category... VMMap’s definition of “Private Data” is more granular than that of
Process Explorer’s “private bytes.” Procexp’s “private bytes” includes
all private committed memory belonging to the process.
i.e. that VMMap's committed "Private Data" should be smaller than procexp's "Private Bytes".
Also, after reading the 'Process committed memory' section of Mark Russinovich's excellent Pushing the Limits of Windows: Virtual Memory, he highlights two cases which won't show up in Private Bytes:
File mapping views with copy-on-write semantics (however, according to VMMap there is no significant space allocated to Mapped Files).
pagefile-backed virtual memory (however, I tried testlimit with the -l flag as suggested, and no significant memory is consumed by pagefile-backed sections)
