mod_xfer_mmapCaveat: This is an experimental module and should be used with care.
The mod_xfer_mmap module was designed to improve the
performance of downloads by using the mmap() function to
map the file being downloaded into memory, and read directly from there,
saving on system calls and buffer copies. The mmap() system call
is available on most modern Unix derivates, but not on all. There are
sometimes system-specific limits on the size and number of files that can be
mmap()d; experimentation is probably the easiest way to find out.
The mod_xfer_mmap module can also create a list of statically
configured files (via TransferMMapFile directives in the main
server configuration) into memory. This mmap()ing is done only at
server start or restart. So whenever one of the mapped files changes on the
filesystem, you must restart the server (by sending it the
HUP signal). To reiterate that point: if the files are modified
in place without restarting the server you may end up serving stale
data. You should update files by unlinking the old copy and putting a new copy
in place. Most tools such as rdist and mv do this.
The reason why this modules doesn't take care of changes to the files is that
doing so would need an extra stat() every time, which is
wasteful and against the intent of I/O reduction.
This module is contained in the mod_xfer_mmap.c file for
ProFTPD 1.2.x, and is not compiled by default. Installation instructions
are discussed here.
The most current version of mod_xfer_mmap can be found at:
http://www.castaglia.org/proftpd/
Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.
2004-10-31: Thanks to Michael Renner for testing out various versions of the module as it was developed.
The TransferMMapEngine directive enables or disables the module's
runtime mapping of files being downloaded. Note that if off,
any files mapped via TransferMMapFile will be unmapped when the
FTP session starts. Thus the daemon process will have mapped memory that
is not used by session processes, which is not an efficient configuration.
The TransferMMapFile directive is used to "preload"
files into memory by mmapping them at startup time, rather than just before
the file is downloaded. They will be automatically unmapped when the server
shuts down. When these mapped files have changed, you should restart the
server to re-map them.
mod_xfer_mmap, copy the mod_xfer_mmap.c
file into:
proftpd-dir/contrib/after unpacking the latest proftpd-1.2 source code. Then follow the usual steps for using third-party modules in proftpd:
./configure --with-modules=mod_xfer_mmap make make install
mod_xfer_mmap. It only affects downloads from the server, not
uploads to the server. If your proftpd uses
sendfile(), that function will take precedence over
mod_xfer_mmap. Files that are handled by
mod_xfer_mmap can still be throttled by the
TransferRate and modules like mod_shaper.
mod_xfer_mmap will not handle a file if the file is larger
than mmap() can handle (which is around 2GB, depending on
your platform's implementation), or if the file is to transfered as an
ASCII file.
Example configuration:
<IfModule mod_xfer_mmap.c>
TransferMMapEngine on
</IfModule>
Note that you may see the size of session processes changing strangely;
this will happen as mod_xfer_mmap maps and unmaps files
from the process' memory space.