Ubuntu Manpage: vmtouch - the Virtual Memory Toucher

NAME

       vmtouch - the Virtual Memory Toucher

SYNOPSIS

           vmtouch [OPTIONS] ... FILES OR DIRECTORIES ...

DESCRIPTION

Portable file system cache diagnostics and control.

vmtouch opens every file provided on the command line and maps it into virtual memory with
mmap(2). The mappings are opened read-only. It recursively crawls any directories and does
the same to all files it finds within them.

With no options, vmtouch will not read from (touch) any memory pages. It will only use
mincore(2) to determine how many pages of each file are actually resident in memory.
Before exiting, it will print a summary of the total pages encountered and how many were
resident.

-t Touch virtual memory pages. Reads a byte from each page of the file. If the page is
not resident in memory, a page fault will be generated and the page will be read from
disk into the file system's memory cache.

Note: Although each page is guaranteed to have been brought into memory, the page
might be evicted from memory by the time the vmtouch command completes.

-e Evict the mapped pages from the file system cache. They will need to be read in from
disk the next time they are accessed. This is the inverse of "-t".

Note: Even if the eviction is successful, pages may be paged back into memory by the
time the vmtouch command completes.

Note: This option is not portable to all systems. See PORTABILITY below.

-l Lock pages into physical memory. This option works the same as "-t" except it calls
mlock(2) on all the memory mappings and doesn't close the descriptors when finished.
At the end of the crawl, if successful, vmtouch will block indefinitely. The files
will be locked in physical memory until the vmtouch process is killed.

Note: While the vmtouch process is holding memory locks, any processes that access the
locked pages will not cause non-resident page faults or address-translation faults
although they may still cause TLB misses.

Note: Because vmtouch holds file descriptors open it may reach the "RLIMIT_NOFILE"
process file descriptor limit. In this case it will try to increase the descriptor
limit which will only work if the process is run with root privileges. Similarly, root
privileges are required to exceed the "RLIMIT_MEMLOCK" limit. Even with root
privileges, "-l" is limited by both the system file descriptor limit and the system
limit on "wired memory".

-L This option is the same as "-l" except that it uses mlockall(2) at the end of the
crawl rather than individually mlock(2)ing each file. Because of this, other unrelated
pages belonging to the vmtouch process will also be locked in memory.

-d Daemon mode. After performing the crawl, disassociate from the terminal and run in the
background as a daemon. This option can only be used with the "-l" or "-L" locking
modes.

-m <max file size>
Maximum file size to map into virtual memory. Files that are larger than this will be
skipped. Examples: 4096, 4k, 100M, 1.5G. The default is 500M.

-p <size range> or <size>
Page mode. Maps the portion of the file specified by a range instead of the entire
file. Size format same as for "-m". Omitted range start (end) value means start (end)
of file. Single <size> value is equivalent to -<size>, i.e. map the first <size>
bytes. Examples: 4k-50k, 100M-2G, -5M, -.

-f Follow symbolic links. With this option, vmtouch will descend into symbolic links that
point to directories and will touch regular files pointed to by symbolic links.
Symbolic link loops are detected and issue warnings.

-F During the crawl, don't recurse into directories that have separate filesystems
mounted on them. This is handy to avoid accidentally touching other filesystems that
have been mounted underneath your target directory.

-i <pattern>
Can be specified multiple times. Ignores files and directories that match any of the
provided patterns. The pattern may include wildcards (remember to escape them from
your shell). This option stops the crawl, so can be used to ignore directories and all
their contents. Example: vmtouch -i .git -i '*.bak' .

-I <pattern>
Can be specified multiple times. Only processes filenames matching one or more of the
provided patterns. The pattern may include wildcards (remember to escape them from
your shell). Example: vmtouch -I '*.c' -I '*.h' .

-b <list file>
The list of files/directories to crawl is read from the specified list file, which by
default should be a newline-separated list, for example the output from the find
command. If the list file is "-" then this list is read from standard input. Example:
find /usr/lib -type f | vmtouch -b -

-0 If -b ("batch mode") is in effect, assume the list file is delimited with NUL bytes
instead of newlines, for example the output from find -print0. This is useful in case
your filenames contain newline characters themselves.

-P <pidfile>
Create a PID file. This option can only be provided in combination with -l or -L. The
PID file will be automatically deleted when vmtouch gets a termination signal (SIGINT,
SIGTERM, SIGQUIT).

-v Verbose mode. While crawling, print out every file being processed along with its
total number of pages and the number of its pages that are currently resident in
memory to standard output.

-q Quiet mode. Suppress the end of crawl summary and all warnings that are normally
printed to standard error. On success print nothing. Fatal errors print a single error
message line to standard error.

-h Normally, if multiple files both point to the same inode then vmtouch will ignore all
but the first it finds so as to avoid double-counting their pages. This option
overrides this behaviour and double-counts anyways.

PORTABILITY

       The page residency summaries depend on mincore(2) which first appeared in 4.4BSD but is
       not present on all unix systems.

       The "-l" and "-L" locking options depends on mlock(2) or mlockall(2), both of which are
       specified by POSIX.1b-1993, Real-Time Extensions.

       The "-e" page eviction option is the least portable. On Linux it uses posix_fadvise(2)
       with "POSIX_FADV_DONTNEED" advice to inform the kernel that the file should be evicted
       from the file system cache. posix_fadvise(2) is specified by POSIX.1-2003 TC1. On FreeBSD,
       the pages are invalidated with msync(2)'s "MS_INVALIDATE" flag. msync(2) is specified by
       POSIX.1b-1993, Real-Time Extensions, although this call is not required to remove pages
       from the file system cache. Some systems like OpenBSD 4.3 don't have posix_fadvise(2),
       don't evict the pages on an msync(2)/"MS_INVALIDATE", and don't evict the pages with
       madvise(2)/"MADV_DONTNEED" so "-e" isn't supported on those systems yet. Using "-e" on
       systems that don't yet support it is a fatal error.

SUPPORTED SYSTEMS

       All vmtouch features have been confirmed to work on the following systems:

       Linux 2.6+
       FreeBSD 4.X
       FreeBSD 7.X
       Solaris 10
       OS X 10.x
       HP-UX 11.31+patches (see below)

       Systems that support everything except eviction:

       OpenBSD 4.3

       CPUs that have been tested:

       x86
       amd64 (x86-64)
       SPARC
       ARMv7

       We would like to support as many systems as possible so please send us any success
       reports, failure reports or patches. Thanks!

SYSTEM NOTES

       Shane Seymour did the HP-UX port and says that either 32-bit or 64-bit binaries can be
       compiled (just use "+DD64" for 64-bit). However, mincore(2) was added to HP-UX 11.31 via
       patches and at least the following patches need to be installed: PHKL_38651, PHKL_38708,
       PHKL_38686, PHKL_38688, and PHCO_38658 (or patches that supersede those ones).

AUTHOR

       Written by Doug Hoyte <doug@hcsw.org>

                                            2021-01-11                                 VMTOUCH(8)