UNIX crawls on EISA system >16MB

UNIX crawls on EISA system >16MB

Post by Don Le » Thu, 12 Dec 1996 04:00:00

We have finally found what was causing our EISA System to slow
to a crawl when adding memory over 16MB and I thought I would
post this so that hopefully others don't get caught like we

The problem was that none of the memory accesses above 16MB
were getting cached in the L2 cache memory, which we figured
fairly quickly and verified with a cachechk program (very
nice utility). cachechk was found in SCO's TLS directory


with the 2 files




The next problem was to figure out why some of our systems
would cache over 16MB and other identical systems would not.

It turns out that the problem was that some of the systems
did not have the EISA configuration utility run on them (this
is a separate utility on diskette that does things other
than the BIOS configuration done by hitting F2/delete
at boot-up).

The BIOS evidentally sees that there is not a valid EISA
configuration and therefore doesn't know what memory above
16MB can be cached since there could be EISA boards with I/O
memory above 16MB. Therefore it caches only the memory below
16MB to be safe.

Our big mistake was when we originally installed these systems
with only 16MB we ignored the BIOS warning "Invalid EISA
configuration" (or something similar) and just went on since
we had no EISA cards and everything worked fine. Not sure how,
but somehow the BIOS warning never showed itself again on boot
up until you do something else with the BIOS (we think
if we go into the BIOS after the warning, the warning

Later when we added 16MB (making the total 32MB) we then started
having a huge slowdown of the system (about 4 times slower than
with only 16MB of memory).

Shame, Shame, Shame on us for ignoring the warning.

Our systems are Intel Deluxe LX systems, don't know if other
systems behave in a similar manner but just want to give others
the info we have learned.



1. EISA/ISA DMA limits for >16MB, is this supported?

I have an EISA motherboard, but all my current controller cards are
ISA.  I expect to add more then 16MB of memory before upgrading the
controller cards to EISA cards.  I did a quick look in both the FAQ and
some of the kernal stuff, but no where did I see anything like the
sysvr4 DMAMAXPAGE parameter that tells sysvr4 to restrict DMA to memory
below 16MB.

Is there a parameter like this in linux?  Can this be done on a per
controller basis, ie EISA disk, but ISA ethernet and serial cards?

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