[CentOS] Re: [which kernel for AMD Sempron processor?

Peter Farrow peter at farrows.org
Mon Oct 3 15:23:02 UTC 2005


Agreed: Should have read "socket A" semprons, since these are the most 
common currently although rapidly disappearing...



>*1* type of Sempron is an Athlon XP processor, the Socket-462
>versions.  With the Socket-754, it is an Athlon 64, although
>most have their 64-bit functions disabled.  Only the lastest,
>specifically branded Sempron 64 have the same, full
>compatibility as their more expensive brothers.




Bryan J. Smith wrote:

>Tom <admin at homemachine.net> wrote:
>  
>
>>Hi all,
>>Is it ok to use the i686 kernel in an AMD Sempron or should
>>I be using another?
>>    
>>
>
>To begin, know there are *3* different types of Semprons ...
>
>1.  All Socket-462 Semprons are 32-bit only
>They are Throughbred/Barton core (last rev Athlon XP)
>
>2.  Most Socket-754 Semprons are 32-bit only
>They are typically Newcastle core (mid-rev Athlon 64) with
>the 52-bit addressing (PAE52) and other protions disabled
>
>3.  New Socket-754 Sempron 64s are PAE52, 64-bit reg, etc...
>Largely as an answer to Intel releasing the P4-Celeron EM64T,
>AMD has stopped disabling the functionality of the cores.  A
>few are even newer Winchester (Rev. D), although I haven't
>seen a Venice (Rev. E -- SSE support) Sempron 64
>
>Now regarding the kernel, know these facts ...
>
>A.  Athlon optimizations -- especially in GCC 3.3 (3.4 seems
>to be much better) -- are not always stable (at least with
>-O3).  So even if you load an Athlon optimized kernel, GCC is
>typically using -O2.  On kernel 2.4, you'll see maybe a 5-15%
>performance improvement.
>
>B.  Linux Kernel 2.6 now adds [32-bit] Athlon optimizations
>as a _dynamic_, modular support in the _stock_ i686 kernel. 
>There is little benefit to building a separate Athlon kernel,
>as I've tried, and seen _no_ noticable improvement.
>
>C.  If you load a x86-64 kernel, most of those _are_ already
>optimized for Athlon architecture because they are the most
>common.  The 32-bit and 64-bit Athlons use the same core
>scheduler design (with only a tiny variance), so just going
>Linux/x86-64 now defaults to Athlon-optmizations.
>
>Personal Note:  I found that rebuilding kernel 2.6 without
>HIGHMEM support (960MiB memory max) increases performance
>well over 15%, and the greatest increase possible with any
>kernel change.
>
>
>Ignacio Vazquez-Abrams <ivazquez at ivazquez.net> wrote:
>  
>
>>The i686 kernel is fine. If you're looking for (small)
>>speedups then you're better off rebuilding the glibc
>>and openssl packages for athlon.
>>    
>>
>
>Agreed.  But be careful in throwing the -O3 switch.  Stick
>with --target=athlon, which the SPEC file should just use -O2
>by default.
>
>
>Peter Farrow <peter at farrows.org> wrote:
>  
>
>>The Sempron is essentislly an Athlon XP processor, so I
>>would use the Athlon kernel...
>>    
>>
>
>*1* type of Sempron is an Athlon XP processor, the Socket-462
>versions.  With the Socket-754, it is an Athlon 64, although
>most have their 64-bit functions disabled.  Only the lastest,
>specifically branded Sempron 64 have the same, full
>compatibility as their more expensive brothers.
>
>
>Peter Farrow <peter at farrows.org> wrote:
>  
>
>>Note:  In some rare cases I have seen Athlon systems hang
>>at boot on i686 kernels, and vice versa, this seems to be
>>system board chipset related.....so don't take out any 
>>kernels you intend to replace until you have seen the
>>system boot reliably..
>>    
>>
>
>There were various errata with Athlons (not always AMD's
>fault), including:  
>- Intel changing the i686 ISA slightly with the P3/P4 (breaks
>even i486 compatibility, sometimes even i586/Pentium).
>- Intel 4M paging mode (AGP coherency issue in the Athlon's
>on-board AGPgart, long, long story -- 4K paging is no lower
>performance on Athlon, so 4M should never be used)
>- ViA ATA and other southbridge peripherals failing
>- SiS I2C compatibility issues
>- Many others
>
>Nothing unheard of and not without some equivalents in the
>Intel world (such as ICH ATA issues), but still notable.
>
>
>Karanbir Singh <mail-lists at karan.org> wrote:
>  
>
>>There is no Athlon kernel in CentOS4.... as Ignacio has
>>pointed out, should you need that small performance boost,
>>then build yourself.
>>    
>>
>
>If you have less than 1GiB RAM, then rebuilding without
>HIGHMEM support would yield a much greater boost.
>
>I had this discussion over on the Fedora-Devel list back
>after Fedora Core 2 came out and I complained about not
>having a separate Athlon kernel.  In the 2.6 kernel, the i686
>kernel dynamically loads support for Athlon optimizations,
>instead of having a separate kernel.
>
>Sure enough, the boost I saw was because I removed HIGHMEM
>support.  I rebuilt the exact same configs with i686 and
>Athlon and saw no difference whatsoever on kernel 2.6. 
>Apparently Linus concept of making optimizations more dynamic
>for various i686 ISA compatible processors (C3/Geode, Athlon,
>even P4, etc...) really works.
>
>
>Tom <admin at homemachine.net> wrote:
>  
>
>>Ah yes, that's why I asked as I couldn't find an Athlon
>>kernel for 4.1 :)  Will there be one?
>>    
>>
>
>Red Hat will not be adding one in the FC2+/RHEL4+ series
>because of their view on the kernel 2.6 modular design. 
>Given my own testing (despite my initial reluctance), they
>are indeed correct.
>
>I'll try to find the post in the LKML (Linux Kernel Mailing
>List) that covers how the dynamic support works in the i686
>kernel.  I know from what I've read before, there was just so
>much overlap combined with so many i686 variants, there was a
>real wish to see a more "core" i686 base with loadable
>optimizations for Geode/C3, Athlon, P4, etc...
>
>
>
>  
>




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