Kernel [memory] tweaking question

[ray at redshift.com]

Great run down, thanks Peter - I will do more checking in those areas.

Ray


At 06:36 PM 4/7/2005 +1000, Peter Jeremy wrote:
| On Tue, 2005-Apr-05 05:26:01 -0700, ray at redshift.com wrote:
| >options		SHMALL=
| >options		SHMMAX=
| >options		SHMMAXPGS=
| >options		SHMMIN=
| >options		SHMMNI=
| >options		SHMSEG=
| 
| These are all reasonably well documented in sys/conf/NOTES.  If you
| want more detail, try a SystemV-oriented Unix book 
| 
| >I understand these control shared memory and how many semaphores the
| >kernel can allocate,
| 
| Close - they only control SystemV shared memory.  Sane shared memory
| is available via mmap(2).  SystemV semaphores are controlled via
| SEMxxx options.  Posix semaphores are listed as 'experimental'.
| 
| > but I guess what I'm not 100% clear on is how
| >the kernel uses these resources when it comes to running something
| >like Apache or MySQL on a heavily loaded server.
| 
| These values all define limits on the amount of shared memory
| available system-wide (SHMALL and SHMMNI) and to a single process
| (remaining options).  As far as I can tell, neither Apache nor MySQL
| use any SystemV IPC on FreeBSD.  (The only thing that I've found that
| does use SHM is X in some modes).
| 
| >  In other words, for
| >something like Apache, how much shared memory is required?
| 
| None.
| 
| >  Or what
| >would adding additional shared memory and/or semaphores provide.
| 
| Nothing.
| 
| >Does the default # provided for by FreeBSD create a problem and/or
| >would it use additional ram if allocated?
| 
| Actually using SystemV IPC will use additional RAM.
| 
| >Is there the anyone on the list that has experience with changing
| >these that can provide a clear, down to earth explanation as to their
| >impacts?
| 
| I have used them on other Unices but never needed to tweak them on
| FreeBSD.   Before adjusting anything, use ipcs(1) to confirm that
| they are being used.  If you seem to be reaching limits (ipcs shows
| that you are close to system limits or the applications are reporting
| allocation errors), then just increase the parameter related to
| whatever you are running out of.
| 
| -- 
| Peter Jeremy
| 
|