Re: Minutes from 10/1 LSE Call

[insecure <insecure@mail.od.ua>]

On Thursday 02 October 2003 22:10, Jeff Garzik wrote:
> insecure wrote:
> > That sounds reasonable, but today's RAM throughput is on the order
> > of 1GB/s, not 100Mb/s. 'Out of L1' theory can't explain 100Mb/s ceiling
> > it seems.
>
> cp(1) data, at least, will never ever be in L1.  Copying data you need
> to look at the ends of the pipeline -- hard drive throughput, PCI bus
> bandwidth, FSB bandwidth, speed at which ext2/3 allocates blocks, and
> similar things are likely bottlenecks.

Hmm.

On Wednesday 01 October 2003 22:19, Hanna Linder wrote:
> We got about 100 mb/sec using the bonie benchmark for block io writes,
> for writes we hit a ceiling around 100-120 mb/sec. Stopped scaling
> after about 3 spindles.
>
> Tried to focus on the block io part of it. Have not tried
> direct or raw io yet.  With block IO we got about 133 mb/sec
> doing a simple dd to dev/null from multiple spindles. This
> was on the 2.6 test 3.
> ....
> Odirect on large block sizes has low cpu utilization ( 3-5% ).
> However with buffered IO we can easily get to 100% cpu utilization.
> If you look at the profile most of that is in the copy_to_user function.

So:
* we hit a ceiling of ~133 Mb/s, no matter how many disks
* CPU utilization is 100%, spent mostly in copy_to_user
* RAM bandwidth is >1Gb/s

These can't be true at once.
At least one of these three statements must be false (imho).
-- 
vda