[ale] Disk performance

[Brian Mathis]

On Sun, Aug 25, 2013 at 1:22 PM, Alex Carver <agcarver+ale at acarver.net>wrote:

> On 8/24/2013 18:03, Ed Cashin wrote:
>
>> On Sat, Aug 24, 2013 at 8:38 PM, Alex Carver <agcarver+ale at acarver.net>**
>> wrote:
>> ...
>>
>>  Well, looks like mostly still working although it has a memory leak
>>> somewhere.  The free memory (according to top) is slowly dropping.  It
>>> went
>>> from 380MB to 365MB in about two hours so 15MB went somewhere during that
>>> time.  Probably one of the cron jobs or apache, just don't know which
>>> yet.
>>>
>>>
>> If you haven't already, you might check whether that memory is simply
>> being
>> used to cache stuff.  Linux aggressively caches blocks from disk in pages
>> of memory, which is usually a convenience.
>>
>
> The free memory went further down by morning but /proc/meminfo seems to
> indicate that most of the used memory is cache.  The two largest processes
> (apache and mysql) haven't changed much in their resident, data, virtual or
> swap usage since last night.  So disk cache it must be.  Any way to tune
> the amount of cache it retains or how often it flushes unused disk pages?
>
> MemTotal:         515356 kB
> MemFree:          170708 kB
> Buffers:           23080 kB
> Cached:           253568 kB
> SwapCached:           44 kB
> Active:           268980 kB
> Inactive:          54828 kB
> Active(anon):       8372 kB
> Inactive(anon):    40504 kB
> Active(file):     260608 kB
> Inactive(file):    14324 kB
>
> AnonPages:         47140 kB
> Mapped:            21844 kB
> Shmem:              1716 kB
> Slab:              15852 kB
> SReclaimable:      12176 kB
> SUnreclaim:         3676 kB
> KernelStack:         776 kB
> PageTables:         1216 kB
>
>

The 'free' command will directly show you how much of the used RAM is in
buffers/cache, and how much is in use by programs.  As far as reducing RAM
used for cache, you almost always don't want to do that.  RAM is both fast
and expensive, so unused RAM is wasted RAM.  Linux is smart enough to be
able to free buffers/cache when a program needs to use it.


❧ Brian Mathis
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