Re: O->A loan

[Thomas Klausner <wiz%NetBSD.org@localhost>]

On Mon, Feb 20, 2012 at 03:52:39AM +0000, YAMAMOTO Takashi wrote:
> > yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi) wrote:
> >> i'd like to to either fix or remove O->A page loaning.
> >> 
> >> the biggest problem is the lack of users.  thus no way to test.
> >> any ideas?
> > 
> > After looking at O->A loaning code in yamt-pagecache branch, looking at
> > the benchmark results by Jaime (some data was sent privately, I think)
> > and contemplating about this mechanism - I tend to think that O->A loaning
> > should be removed.  It is a significant code complexity in UVM with some
> > special casing and benchmarks indicate that the benefit is somewhat small.
> > 
> > Does not seem to be worth, does it?
> 
> with this particular use alone, i tend to agree.
> 
> fwiw, the following is probably one of the best cases for loaned read.
> 
> YAMAMOTO Takashi
> 
> without loaned read:
> 
>       kuma% dd if=/netbsd bs=1m|wc -c
>       14+1 records in
>       14+1 records out
>       14768691 bytes transferred in 0.038 secs (388649763 bytes/sec)
>        14768691
> 
> with loaned read:
> 
>       kuma% dd if=/netbsd bs=1m|wc -c
>       14+1 records in
>       14+1 records out
>       14768691 bytes transferred in 0.023 secs (642117000 bytes/sec)
>        14768691

Can you please tell in a few words what O->A loaning is, and why it speeds up 
this use case?

Thanks,
 Thomas