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Top Forums UNIX for Advanced & Expert Users Help optimizing sort of large files Post 302924787 by DGPickett on Wednesday 12th of November 2014 11:23:04 AM
Old 11-12-2014
There is no swap space use with mmap() unless you choose the more exotic options. The basic mmap() swaps in file pages directly to RAM, giving you a way to work huge files with all of RAM as a cache. If you overmap a file, you can write pages. Empty pages can exist, supporting sparse matrices. This is a very powerful door into application use of VM and full exploitation of RAM.

Sorting small into larger in steps allows the problem to periodically expand to the next level of storage: from the caches to the RAM to the disk. The only flaw is that it might be good to merge more than 2 at a time, so when you get to the disk size merges, there is just one or at least fewer. The merge could use some sort of comparison tournament tree to do the merge, sized to fit in L1 cache, where each replacement item needs to be fit into the tree before using the smallest from the tree. Note that each item will take a whole page of RAM, or two, and N-N+2 lines of L1 cache, depending on the cache and item sizes and page/line offsets. Trying to fill L1 cache is fraught with difficulty, as many items may demand the same cache bucket, so it pays to shoot a bit low, like 1/4 - 1/2 L1 cache size.
 

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FS_SETCACHESIZE(1)					       AFS Command Reference						FS_SETCACHESIZE(1)

NAME
fs_setcachesize - Sets the size of the disk cache SYNOPSIS
fs setcachesize [-blocks <size (0 = reset)>>] [-reset] [-help] fs setca [-b <size (0 = reset)>>] [-r] [-h] fs cachesize [-b <size (0 = reset)>>] [-r] [-h] fs ca [-b <size (0 = reset)>>] [-r] [-h] DESCRIPTION
The fs setcachesize command changes the number of kilobyte blocks of local disk space available to the Cache Manager for its data cache, on machines that use a disk cache. The command is not operative on machines that use a memory cache. To return the cache size to the default value specified in the third field of the local /etc/openafs/cacheinfo file, provide a value of 0 to the -blocks argument. To return the cache size to the value set when the machine was last rebooted, use the -reset flag instead of the -blocks argument. This is normally the amount specified in the cacheinfo file, unless the -blocks argument was included on the afsd command to override the cacheinfo value. To display the current cache size and amount of cache in use, for both disk and memory caches, use the fs getcacheparms command. CAUTIONS
This command is not operative on machines using a memory cache, and results in an error message. To change memory cache size, edit the cacheinfo file and reboot, or reboot and provide the -blocks argument to the afsd command. On machines using a disk cache, do not set the cache size to exceed 85% to 90% of the actual disk space available for the cache directory. The cache implementation itself requires a small amount of space on the partition. OPTIONS
-blocks <size> Specifies the amount of disk space available for the Cache Manager to devote to the cache. The size should be a positive integer followed by an optional suffix: "K" for kibibytes (1024 bytes, the default), "M" for mebibytes (1024 kibibytes), "G" for gibibytes (1024 mebibytes), and "T" for tebibytes (1024 gibibytes). Provide a value of 0 to set cache size to the default specified in the cacheinfo file. -reset Returns the cache size to the value set when the machine was last booted. This agrees with the value in the cacheinfo file unless the -blocks argument was used on the afsd command. -help Prints the online help for this command. All other valid options are ignored. EXAMPLES
The following command sets the disk cache size to 25000 kilobyte blocks. % fs setcachesize -blocks 25000 Both of the following commands reset the disk cache size to the value in the cacheinfo file, assuming that the -blocks argument to the afsd command was not used. % fs setcachesize -blocks 0 % fs setcachesize -reset PRIVILEGE REQUIRED
The issuer must be logged in as the local superuser root. SEE ALSO
cacheinfo(5), afsd(8), fs_getcacheparms(1) COPYRIGHT
IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved. This documentation is covered by the IBM Public License Version 1.0. It was converted from HTML to POD by software written by Chas Williams and Russ Allbery, based on work by Alf Wachsmann and Elizabeth Cassell. OpenAFS 2012-03-26 FS_SETCACHESIZE(1)
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