Skip to content

DOCS-511 rewording page fault entries. virtual memory -> disk #212

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 1 commit into from
Sep 11, 2012
Merged

DOCS-511 rewording page fault entries. virtual memory -> disk #212

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
15 changes: 8 additions & 7 deletions source/faq/storage.txt
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -41,22 +41,22 @@ MongoDB uses memory mapped files for managing and interacting with all
data. MongoDB memory maps data files to memory as it accesses
documents. Data that isn't accessed is *not* mapped to memory.

.. _faq-storage-page-faults:

What are page faults?
---------------------

Page faults will occur if you're attempting to access some part of a
Page faults will occur if you're attempting to access part of a
memory-mapped file that *isn't* in memory.

If there is free memory, then the operating system can find the page
on disk and load it to memory directly. However, if there is no free
memory, the operating system must:

- find a page in memory that is stale or no longer needed,

- if needed, the operating system might need to write this page to
disk before removing it from memory.
- find a page in memory that is stale or no longer needed, and write
the page to disk.

- read the page from disk and load it into memory.
- read the requested page from disk and load it into memory.

This process, particularly on an active system can take a long time,
particularly in comparison to reading a page that is already in
Expand All @@ -69,7 +69,8 @@ What is the difference between soft and hard page faults?
data that isn't currently in active memory. A "hard" page fault
refers to situations when MongoDB must access a disk to access the
data. A "soft" page fault, by contrast, merely moves memory pages from
one list to another, and does not require as much time to complete.
one list to another, such as from an operating system file
cache. In production, MongoDB will rarely encounter soft page faults.

What tools can I use to investigate storage use in MongoDB?
-----------------------------------------------------------
Expand Down
5 changes: 4 additions & 1 deletion source/reference/glossary.txt
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -802,7 +802,10 @@ Glossary
page fault
The event that occurs when a process requests stored data
(i.e. a page) from memory that the operating system has moved to
virtual memory.
disk.

.. seealso:: :ref:`Storage FAQ: What are page faults?
<faq-storage-page-faults>`

working set
The collection of data that MongoDB uses regularly. This data
Expand Down