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Top Forums Programming Shared memory in shared library Post 302119366 by DreamWarrior on Tuesday 29th of May 2007 01:07:18 PM
Old 05-29-2007
Quote:
Originally Posted by Perderabo
I do not share your trepidation regarding the performance hit. This is virtually the definition of of an array reference is performed and I use arrays quite a bit. Switching your app entirely to arrays and never using pointers at all might actually improve performance provided that you use the optimizer. In any event, many implementations to not allow you to choose the address of a shared memory segment and portable code should not rely on having that option. Shared libraries are compiled using PIC (position independent code) despite the fact that there is often a minor performance hit with PIC. Shared data segments should also be position independent. It's the cost of doing business.
That works beautifully if I want to partition the shared memory up into several buckets and reference each bucket by its index. However, assuming the DB is made up of differently sized information, I must either pick a bucket size big enough to store anything (and waste space on smaller things) or I allocate dynamically sized buckets and pass around pointers (as indexes no longer work).

Essentially, I was thinking I could create a version of malloc that operated within a shared memory region and then use it to allocate items in the DB dynamically to be stored in a chained hash table.

The "performance hit" on pointers is that I need to store the "pointer" to the bucket that I allocated (via my malloc routine) in shared memory somehow. Either that pointer is a native pointer into shared memory, or it is an offset into shared memory that every time application code goes to access a pointer it'll need to perform a conversion routine against it to acquire its position independant address. This would be required for either the array or non-native pointer methods. I guess I could tell the application (in the non-native pointer method) that the shared memory is a huge array of characters and access pointers through an "index" into the character array cast to the appropriate data type...but this seems just as ugly.

Maybe creating an intermediate "malloc" library against a shared memory segment is silly...but I don't know of a better way to store various dynamically sized data into any memory segment without wasting space on static sized buckets.

Last edited by DreamWarrior; 05-29-2007 at 02:18 PM..
 

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plock(2)							System Calls Manual							  plock(2)

NAME
plock() - lock process, text, data, stack, or shared library in memory SYNOPSIS
DESCRIPTION
The system call allows the calling process to lock the text segment of the process (text lock), its data segment (data lock), or both its text and data segment (process lock) into memory. Stack segments are also locked when data segments are locked. Shared library text and shared library data segments (shlib lock) can also be locked. Locked segments are immune to all routine swapping. also allows these seg- ments to be unlocked. The effective user ID of the calling process must be a user with the privilege. op must be one of the following: Lock text and data segments into memory (process lock) Lock text segment into memory (text lock) Lock data segment into memory (data lock) Remove locks Lock shared library text and shared library data segments (shared library lock) Lock text, data and shared library text and shared library data segments into memory (process and shared library lock) Lock text, shared library text and shared library data segments into memory (text and shared library lock) Lock data, shared library text and shared library data segments into memory (data and shared library lock) Although and the family of functions may be used together in an application, each may affect the other in unexpected ways. This practice is not recommended. Security Restrictions Some or all of the actions associated with this system call require the privilege. Processes owned by the superuser have this privilege. Processes owned by other users may have this privilege, depending on system configuration. See privileges(5) for more information about privileged access on systems that support fine-grained privileges. RETURN VALUE
returns the following values: Successful completion. Failure. The requested operation is not performed. is set to indicate the error. ERRORS
If fails, is set to one of the following values. op is equal to and a process lock, a text lock, or a data lock already exists on the calling process. op is equal to and a text lock or process lock already exists on the calling process. op is equal to and a data lock, or process lock already exists on the calling process. op is equal to and no type of lock exists on the calling process. op is equal to and there are no unlocked shared library segments in the calling process. op is equal to and a process lock, a text lock, or a data lock already exists on the calling process. op is equal to and a text lock or process lock already exists on the calling process. op is equal to and a data lock, or process lock already exists on the calling process. op is not equal to one of the values specified in is not allowed in a window. See vfork(2). There is not enough lockable memory in the system to satisfy the locking request. The effective user ID of the calling process is not a user with the privilege. EXAMPLES
The following call to locks the calling process in memory: SEE ALSO
setprivgrp(1M), exec(2), exit(2), fork(2), getprivgrp(2), mlock(2), vfork(2), privileges(5). STANDARDS CONFORMANCE
plock(2)
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