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Implement probabilistic reader/writer locks under windows so that windows

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can have multiple simultaneous readers. (CVS 714)

FossilOrigin-Name: 2127de3f03537ef2f18120e773f7603e0984ff81
This commit is contained in:
drh
2002-08-14 12:56:54 +00:00
parent c330af1dca
commit d1efac52d5
6 changed files with 96 additions and 38 deletions
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22
www/faq.tcl
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@@ -1,7 +1,7 @@
#
# Run this script to generated a faq.html output file
#
set rcsid {$Id: faq.tcl,v 1.15 2002/08/13 20:45:41 drh Exp $}
set rcsid {$Id: faq.tcl,v 1.16 2002/08/14 12:56:56 drh Exp $}
puts {<html>
<head>
@@ -169,19 +169,27 @@ faq {
application access a single database file at the same time?
} {
<p>Multiple processes can have the same database open at the same
time. On unix systems, multiple processes can be doing a SELECT
time. Multiple processes can be doing a SELECT
at the same time. But only one process can be making changes to
the database at once. On windows, only a single process can be
reading from the database at one time since Win95/98/ME does not
support reader/writer locks.</p>
the database at once.</p>
<p>Win95/98/ME lacks support for reader/writer locks in the operating
system. Prior to version 2.7.0, this meant that under windows you
could only have a single process reading the database at one time.
This problem was resolved in version 2.7.0 by implementing a user-space
probabilistic reader/writer locking strategy in the windows interface
code file. Windows
now works like Unix in allowing multiple simultaneous readers.</p>
<p>The locking mechanism used to control simultaneous access might
not work correctly if the database file is kept on an NFS filesystem.
You should avoid putting SQLite database files on NFS if multiple
processes might try to access the file at the same time.</p>
processes might try to access the file at the same time. On Windows,
Microsoft's documentation says that locking may not work under FAT
filesystems if you are not running the Share.exe daemon.</p>
<p>Locking in SQLite is very course-grained. SQLite locks the
entire database. Big database servers (PostgreSQL, MySQL, Oracle, etc.)
entire database. Big database servers (PostgreSQL, Oracle, etc.)
generally have finer grained locking, such as locking on a single
table or a single row within a table. If you have a massively
parallel database application, you should consider using a big database