[Novalug] Interesting memory usage on big server

[Ben Creitz]

On 9/5/07, Kyle Powell <kyle_powell at yahoo.com> wrote:
> --- Ben Creitz <creitz at gmail.com> wrote:
>
> > The other day I noticed that out of the 32 GB of RAM on the physical
> > host, only about 15 GB were "free."  It happened to be a good time for
> > me to apply some Dell and RH updates and reboot... after the reboot I
> > watched with vmstat or 'free' as the available memory slowly trickled
> > away... for example this morning 'free' reported 28.9 GB available,
> > this afternoon only 27.34 GB.  top's results, sorted by memory, do not
> > show any processes using much... certainly not on the order of several
> > GBs.  So I opened a ticket with RH.
>
> Look closely at the output of the "free" command:
> [root at foo ~]# free
>              total       used       free     shared    buffers     cached
> Mem:        384156     378960       5196          0       5868     294996
> -/+ buffers/cache:      78096     306060
> Swap:       522104          0     522104
>
> While the output shows I have ~5MB free out of the 384MB of RAM in this system,
> it also shows I have 5MB allocated to buffers and 294MB in pagecache. Cache
> memory can be freed quite easily should the system come under memory pressure.
> If you look at the second line, which gives a better picture of the memory
> situation by showing used and free memory +/- buffers and cache, you'll see
> that only 78MB of memory is in use and 300MB+ is actually available should the
> kernel or another program suddenly need more memory. If I looked through the
> memory used by processes listed in top, I would probably come up with a number
> slightly larger than 78MB (due to the way top displays shared memory usage).
>
> The philosophy behind this is "if you have memory in the system, it should be
> used", even if it's just to cache data read from disk. In the event that same
> data is needed again, it's much faster to fetch it from memory than to retrieve
> it from the disk again. It's fast and easy enough to free it should something
> else need the memory in the future. HTH.
>
> -Kyle

I'm glad I posted this to the list... I have learned bunch from you
all.  Thanks!

Ben