[mdlug] Moving to Debian 64bit - Lessons Learned?

[Brian]

Since it can't access all of the memory directly through the cpu the
kernel has to create a structure to map requests to the higher memory
addresses.  This structure has to be stored in regions that are
directly addressible by the cpu.  If you take a look at /proc/meminfo
this is displayed as low mem and high mem(addressed virtually).  For
it's work the kernel can only operate in low mem.  Because of this
it's actually quite possible to crush an OS's performance on 32 bit
hardware by having too much memory.

--
Brian

On 8/1/09, Michael S. Mikowski <z_mikowski at yahoo.com> wrote:
> On Friday 31 July 2009 04:02:42 am Stan Green wrote:
>> I am going to attempt to move my main desktop PC to Debian 5.0.2 64bit.
>> (I'm doing it on a different PC, so I can keep my current one running
>> until
>> I have the new one running correctly.)  This is my first attempt at a
>> 64bit
>> Linux install. Are there any lessons others have learned from making this
>> switch or with Debian 64bit? Has anyone had success/failure with VMWare
>> Workstation 5.5 in this configuration?
>>
>> FYI: My main reason to go to 64bit is to be able to access more than 3gb
>> of
>> memory. Running VMWare can quickly chew up memory.
>>
>
> My advice is don't do it.  Really, all the headaches you get from 64 bit are
> not made up with speed or memory except in instances that usually occur only
> in very high-volume database or number crunching situations.
>
> First, speed:  http://www.phoronix.com/ has shown repeatedly that 64 bit
> desktops rarely run faster for most desktop tasks.  In fact, for most apps,
> 64
> bit often runs slower.
>
> Second, memory: You can install a 32 bit kernel with large memory support.
> Last year we installed 2 Dell 1950's with 8 64 bit cores and 16G ram each to
> handle ~ 3 billion MySQL inserts and ~750 million reads per day.  We
> intended
> to move it to a 64 bit OS to access all that memory.
>
> However, we discovered using the 32 bit PAE kernel provided by RedHat (RHEL
> 5)
> handled the memory just fine.  A check of free or top showed all 16G
> available.
> Exactly how that memory was accessed (e.g. if it were "paged" or not) I
> never
> investigated since MySQL ran like a top.
>
> <http://www.cyberciti.biz/tips/redhat-enterprise-linux-4gb-plus-ram-
> support.html#comments>
>
> So unless you are doing some serious crunching, 64 bit is probably simply
> not
> compelling.  Avoiding all the hassle of 32 bit library and driver
> incompatibilities probably is :)
>
>
> Cheers, Mike
>
> ps Last I checked (8 months ago) VMWare only supported 32 bit kernels for
> the
> host OS.  So unless that has changed, there is little benefit there either.
> _______________________________________________
> mdlug mailing list
> mdlug at mdlug.org
> http://mdlug.org/mailman/listinfo/mdlug
>