Re: Known problems with dcache?

[Andrew Cagney <ac131313@redhat.com>]

> On Fri, Jan 10, 2003 at 05:35:56PM -0500, Andrew Cagney wrote:
> 
>> 
> 
>> >We don't use the straw on some targets now; Linux (need to get back to
>> >that patch and turn it on always!), *BSD.
> 
>> 
>> Right so that 32 byte read is now cheap.
>> 
> 
>> >The dcache needs some serious work if you want it to be always on. 
>> >Last time I tested it it caused an actual slowdown.  Basically, it's
>> >too small to be useful.
>> >
>> >#define DCACHE_SIZE 64
>> >#define LINE_SIZE_POWER (5)
>> >
>> >So it never stores more than 2K.  LinuxThreads _overwhelms_ that, by a
>> >downright boggling amount.
> 
>> 
>> You wouldn't know why it caused a slow down?  Th 32 byte read should now 
>> be cheaper.
> 
> 
> This was before the 32-byte-read support.  So we read lines in, did
> more computation than before, and overwrote them in the cache; exactly
> the same I/O, more CPU.

Well, possibly more I/O.  And, I think, the I/O / context switch / ... 
is the expensive part.

>   Might be worth trying it again, with larger
> lines, now.  Not sure.

Was it on an i386?  If it was, the other other cache would easily skew 
any results.

Andrew