Wednesday 29 January 2014

Types of Page Faults

This blog post will expand upon the idea of Page Faults, which resolve problems with Virtual to Physical Address Translation, and take a look at the different kinds of Page Faults which can happen.

Collided Page Faults

Collided Page Faults are common on modern systems which have multiple threads. A Collided Page Fault is a situation whereby a thread within the same process or from a different process, causes a Page Fault on a page which is already paged-in by another thread of the same process or a different thread from a different process as mentioned before.

Before reading the rest of this section, the concept of Transition will apply to the PFN Database and not to PTEs and Prototype PTEs. Transition in terms of the PFN Database, will mean that the page isn't in a working set list or present on any paging lists, therefore the page will not be owned by anyone and will be in Transition when a I/O operation is being performed on it.

I haven't been able to find the correct symbols for the Transition Page List Head, and therefore won't be able to apply the _MMPFNLIST data structure to it. The symbol is most likely only accessible with Private symbols.

The ListName member is a enumeration which contains a list of all the PFN list types.

The Pager knows if the page is being read by checking the ReadInProgress bit is set in the _MMPFNENTRY data structure.

From here, the Pager may issue a Event dispatcher object (thread which created the fault owns the Event object) to cause other threads to wait, or alternatively it may create a parallel I/O operation to prevent deadlocks occurring in the filesystem, which will cause two threads to compete against each other, with the thread which is able to complete the I/O operation first being able to resolve the Page Fault.

Once the I/O has completeted, the ReadInProgress bit will be cleared and the PTE will be updated to point to the physical page. Any other threads which wish to obtain the PFN Database lock as seen in the _MMPFNLIST data structure, will check to see if any previous I/O operations completed successfully by checking the InPageError bit.

Clustered Page Faults

Clustered Page Faults are designed for performance, rather to resolve problems like deadlocks with the file system mechanisms being used. Clustered Page Faults rely upon placing large numbers of pages into the page cache, rather than the system working set in order to reduce the consumption of the virtual address space, and to avoid the limiting factor of the virtual address space size for the given system.

Furthermore, any pages which will be reused, do not have to invalidated within the TLB Cache with IPI's or flushing and rewriting of the CR3 register. Any PTE's which had mapping, will have the Transition bit set and placed onto a Standby list.

If the page is then referenced by a process, it is added to the working set of that process, however, if the page is never referenced it will simply remain within the Page Cache. Any pages which are present in the Page Cache and physical memory, will be represented with a dummy page. With Page Clustering, if two pages are referenced within the same cluster, then the Memory Manager will issue a single I/O operation on the two pages, instead of two read operations to increase performance.

The other pages which are already present in physical memory will point to a systemwide dummy page.

The Size field indicates the size of the MDL data structure and the size of the array of PFN entries for the resident pages.

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