The madv.so.1 shared object provides a means by which the VM advice can be selectively configured for a launched process (or processes) and its descendants. To enable madv.so.1, the following string needs to be present in the environment (see ld.so.1(1)) along with one or more MADV environment variables:
If the madv.so.1 shared object is specified in the LD_PRELOAD list, the following environment variables are read by the madv shared object to determine to which created process(es) to apply the specified advice.
MADV specifies the VM advice to use for all heap, shared memory, and mmap regions in the process address space. This advice is applied to all created processes.
Values for advice correspond to values in <sys/mman.h> used in madvise(3C) to specify memory access patterns:
config-file is a text file which contains one or more madv configuration entries of the form:
Advice specified in config-file takes precedence over that specified by the MADV environment variable. When MADVCFGFILE is not set, advice is taken from file /etc/madv.conf if it exists.
exec-name specifies the name of an application or executable. The corresponding advice is set for newly created processes (see getexecname(3C)) that match the first exec-name found in the file.
exec-name can be a full pathname, a base name, or a pattern string. See File Name Generation in sh(1) for a discussion of pattern matching.
exec-args is an optionally specified pattern string to match against arguments. Advice is set only if exec-args is not specified or occurs within the arguments to exec-name.
advice-opts is a comma-separated list specifying the advice for various memory region(s):
madv=advice Applies to all heap, shared memory, and mmap regions in the process address space.
heap=advice The heap is defined to be the brk area (see brk(2)). Applies to the existing heap and for any additional heap memory allocated in the future.
dism=advice Shared memory segments (see shmat(2)) attached using any flags, flag SHM_SHARE_MMU, or flag SHM_PAGEABLE respectively. Options ism and dism take precedence over option shm.
mapanon=advice Mappings established through mmap(2) using any flags, flag MAP_SHARED, flag MAP_PRIVATE, or flag MAP_ANON, respectively. Options mapshared, mapprivate, and mapanon take precedence over option map. Option mapanon takes precedence over mapshared and mapprivate.
By default, error messages are logged via syslog(3C) using level LOG_ERR and facility LOG_USER. If MADVERRFILE contains a valid pathname (such as /dev/stderr), error messages will be logged there instead.
Example 1: Applying advice to all ISM segments
The following configuration applies advice to all ISM segments for application /usr/bin/foo:
example$ LD_PRELOAD=$LD_PRELOAD:madv.so.1 example$ MADVCFGFILE=madvcfg example$ export LD_PRELOAD MADVCFGFILE example$ cat $MADVCFGFILE /usr/bin/foo:ism=access_lwp
Example 2: Setting advice for all applications with exception
The following configuration sets advice for all applications with the exception of ls.
example$ LD_PRELOAD=$LD_PRELOAD:madv.so.1 example$ MADV=access_many example$ MADVCFGFILE=madvcfg example$ export LD_PRELOAD MADV MADVCFGFILE example$ cat $MADVCFGFILE ls:
Example 3: Precedence rules (continuation from Example 2)
Because MADVCFGFILE takes precedence over MADV, specifying '*' (pattern match all) for the exec-name of the last madv configuration entry would be equivalent to setting MADV. The following is equivalent to example 2:
example$ LD_PRELOAD=$LD_PRELOAD:madv.so.1 example$ MADVCFGFILE=madvcfg example$ export LD_PRELOAD MADVCFGFILE example$ cat $MADVCFGFILE ls: *:madv=access_many
Example 4: Applying advice for different regions
The following configuration applies one type of advice for mmap regions and different advice for heap and shared memory regions for a select set of applications with exec names that begin with foo:
example$ LD_PRELOAD=$LD_PRELOAD:madv.so.1 example$ MADVCFGFILE=madvcfg example$ export LD_PRELOAD MADVCFGFILE example$ cat $MADVCFGFILE foo*:madv=access_many,heap=sequential,shm=access_lwp
Example 5: Applying advice selectively
The following configuration applies advice for the heap of applications beginning with ora that have ora1 as an argument:
example$ LD_PRELOAD=$LD_PRELOAD:madv.so.1 example$ MADVCFGFILE=madvcfg example$ export LD_PRELOAD MADVCFGFILE example$ cat $MADVCFGFILE ora* ora1:heap=access_many
/etc/madv.conf Configuration file
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|
cat(1), ld.so.1(1), proc(1), sh(1), brk(2), exec(2), fork(2), mmap(2), memcntl(2), shmat(2), getexecname(3C), madvise(3C), syslog(3C), proc(4), attributes(5)
The advice is inherited. A child process has the same advice as its parent. On exec() (see exec(2)), the advice is set back to the default system advice unless different advice has been configured via the madv shared object.
Advice is only applied to mmap regions explicitly created by the user program. Those regions established by the run-time linker or by system libraries making direct system calls (for example, libthread allocations for thread stacks) are not affected.