According to general notions about the page cache and this answer the system file cache essentially uses all the RAM not used by any other process. This is, as far as I know, the case for the page cache in Linux.
Since the notion of "free RAM" is a bit blurry in Windows, my question is, what part of the RAM does the system file cache use? For example, is the same as "Available RAM" in the task manager?
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Your answer is not entirely true.
The file cache, also called the system cache, describes a range of virtual addresses, it has a physical working set that is tracked by MmSystemCacheWs, and that working set is a subset of all the mapped file physical pages on the system.
The system cache is a range of virtual addresses, hence PTEs, that point to mapped file pages. The mapped file pages are brought in by a process creating a mapping or brought in by the system cache manager in response to a file read.
Existing pages that are needed by the file cache in response to a read become part of the system working set. If a page in a mapped file is not present then it is paged in and it becomes part of the system working set. When a page is in more than one working set (i.e. system and a process or process and another process), it is considered to be in a shared working set on programs like VMMap.
The actual mapped file pages themselves are controlled by a section object, one per file, a data control area (for the file) and subsection objects for the file, and a segment object for the file with prototype PTEs for the file. These get created the first time a process creates a mapping object for the file, or the first time the system cache manager creates the mapping object (section object) for the file due to it needing to access the file in response to a file IO operation performed by a process.
When the system cache manager needs to read from the file, it maps 256KiB views of the file at a time, and keeps track of the view in a VACB object. A process maps a variable view of a file, typically the size of the whole file, and keeps track of this view in the process VAD. The act of mapping the view is simply filling in PTEs to point to physical pages that contain the file that are already resident by looking at the prototype PTE for that range in the file and seeing what it contains, and in the event that the prototype PTE does not point to a physical page, initialising the PTE to point to the prototype PTE instead of the page it points to, and the PTE is left invalid, and this fault will be resolved on demand on a page by page basis when the read from the view is actually performed.
The VACBs keep track of the 256KiB views of files that the cache manager has opened and the virtual address range of that view, which describes the range of 64 PTEs that service that range of virtual addresses. There is no virtual external fragmentation or page table external fragmentation as all views are the same size, and there is no physical external fragmentation, because all pages in the view are 4KiB. 256KiB is the size chosen because if it were smaller, there would be too many VACB objects (64 times as many, taking up space), and if it were larger, there would effectively be a lot of internal fragmentation from reads and hence large virtual address pollution, and also, the VACB uses the lower bits of the virtual address to store the number of I/O operations that are currently being performed on that range, so the VACB size would have to be increased by a few bits or it would be able to handle fewer concurrent I/O operations.
If the view were the whole size of the file, there would quickly be a lot of virtual address pollution, because it would be mapping in the whole of every file that is read, and file mappings are supposed to be for user processes which knowingly map a whole file view into its virtual address space, expecting the whole of the file to be accessed. There would also be a lot of virtual external fragmentation, because the views wouldn't be the same size.
As for executable images, they are mapped in separately with separate prototype PTEs and separate physical pages, separate control area, separate segment and subsection object to the data file map for the file. The process maps the image in, but the kernel also maps images for ntoskrnl.exe, hal.dll in large pages, and then driver images are on the system PTE working set.
Yes, the RAM used by the file cache is essentially the RAM displayed as available in the Task Manager. But not exactly. I'll go into details and explain how to measure it more precisely.
The file cache is not a process listed in the list of processes in the Task Manager. However, since Vista, its memory is managed like a process. Thus I'll explain a bit of memory management for processes, the file cache being a special case.
In Windows, the RAM used by a process has essentially two states: "Active" and "Standby":
"Standby" + "Free" RAM is what is called "Available" in the task manager. "Free" RAM tends to be near 0 in Windows but you can meaningfully consider Standby RAM is free as well.
Standby RAM is considered as "not used for a while by the process". It is the part of the RAM that will be used to give new memory to processes needing it. But it still belongs to the process and could be used directly if the owning process suddenly access it (which is considered as unlikely by the system).
Thus the file cache has "Active" RAM and "Standby" RAM. "Active" RAM is somehow the cache for data recently accessed. "Standby" RAM is the cache for data accessed a while ago. The "Active" RAM of the file cache is usually relatively small. The Standby RAM of the file cache is most often all the RAM of your computer: Total RAM - Active RAM of all processes. Indeed, other processes rarely have Standby RAM because it tends to go to the file cache if you do disk I/O quite a bit.
This is the info displayed by RAMMap for a busy server doing a lot of I/O and computation:
The file cache is the second row called "Mapped file". See that most of the 32 GB is either in the Active part of other processes, or in the Standby part of the file cache.
So finally, yes, the RAM used by the file cache is essentially the RAM displayed as available in the Task Manager. If you want to measure with more certainty, you can use RAMMap.