Re: ppc44x - how do i optimize driver for tlb hits

[Josh Boyer <jwboyer@linux.vnet.ibm.com>]

On Fri, Sep 24, 2010 at 02:43:52PM +1000, Benjamin Herrenschmidt wrote:
>> The DMA is what I use in the "real world case" to get data into and out 
>> of these buffers.  However, I can disable the DMA completely and do only
>> the kmalloc.  In this case I still see the same poor performance.  My
>> prefetching is part of my algo using the dcbt instructions.  I know the
>> instructions are effective b/c without them the algo is much less 
>> performant.  So yes, my prefetches are explicit.
>
>Could be some "effect" of the cache structure, L2 cache, cache geometry
>(number of ways etc...). You might be able to alleviate that by changing
>the "stride" of your prefetch.
>
>Unfortunately, I'm not familiar enough with the 440 micro architecture
>and its caches to be able to help you much here.

Also, doesn't kmalloc have a limit to the size of the request it will
let you allocate?  I know in the distant past you could allocate 128K
with kmalloc, and 2M with an explicit call to get_free_pages.  Anything
larger than that had to use vmalloc.  The limit might indeed be higher
now, but a 4MB kmalloc buffer sounds very large, given that it would be
contiguous pages.  Two of them even less so.

>> Ok, I will give that a try ... in addition, is there an easy way to use
>> any sort of gprof like tool to see the system performance?  What about
>> looking at the 44x performance counters in some meaningful way?  All
>> the experiments point to the fetching being slower in the full program
>> as opposed to the algo in a testbench, so I want to determine what it is
>> that could cause that.
>
>Does it have any useful performance counters ? I didn't think it did but
>I may be mistaken.

No, it doesn't.

josh