DRAM thermal throttling by 440BX chipset

Josip Loncaric (josip@icase.edu)
Tue, 16 Nov 1999 13:08:24 -0500

The 440BX motherboard chipset can block DRAM read/write accesses under
certain conditions. This behavior is not under direct CPU control, and
it may have unintended consequences, particularly where device drivers
are concerned. On our system, some DRAM writes may be delayed by almost
20 microseconds, which may explain why sometimes I/O instructions can
overtake writes to RAM.

For example, on our system the motherboard chipset counts DRAM writes
over a window of one second. When more than 213065 QWords are written
during that second, the 440BX chipset starts a throttle monitoring
window. After 627 QWords are written during this period, further write
requests are blocked for the remainder of this 20 microsecond window.
This monitoring is repeated for over a minute. Reads are handled
similarly.

Thanks to a code segment by Robert Redelmeier, I managed to examine the
440BX chipset registers which control the DRAM read/write thermal
throttling behavior. Here is my interpretation of the results obtained
using our ASUS P2B-DS motherboards (BIOS 1009, Linux Kernel 2.2.13, dual
Pentium III 500MHz):

DWTC: 80003e8f 7fffb39c
Throttle Lock ON: DWTC is read-only
Global DRAM Write Sampling Window: 1000 milliseconds
Global QWord Threshold: 213065 QWords per 1000ms
Throttle Time: 63000 milliseconds
Throttle Monitoring Window: 2000 DRAM CLKs
Throttle QWord Maximum: 627 QWords in 2000 DRAM CLKs while throttling

DRTC: 00003e9d cff7d32c
Throttle Lock ON: DRTC is read-only
Global DRAM Read Sampling Window: 1000 milliseconds
Global Read QWord Threshold: 189845 QWords per 1000ms
Read Throttle Time: 63000 milliseconds
Read Throttle Monitoring Window: 992 DRAM CLKs
Read Throttle QWord Maximum: 613 QWords in 992 DRAM CLKs while
throttling

Our other ASUS motherboards (P2B and P2B-D) are similar except that some
have a bit lower threshold values.

Sincerely,
Josip

-- 
Dr. Josip Loncaric, Senior Staff Scientist        mailto:josip@icase.edu
ICASE, Mail Stop 132C                       http://www.icase.edu/~josip/
NASA Langley Research Center             mailto:j.loncaric@larc.nasa.gov
Hampton, VA 23681-2199, USA    Tel. +1 757 864-2192  Fax +1 757 864-6134


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