Subject: Re: Unified Buffer Cache 1st snapshot
To: None <>
From: Thor Lancelot Simon <>
List: tech-kern
Date: 09/21/1998 20:27:33
On Mon, Sep 21, 1998 at 11:25:33AM +0200, Stefan Grefen wrote:
> In message <>  Chuck Silvers wrote:
> > hi folks,
> > 
> [...]
> > the interfaces for the kernel to access the page cache are ubc_alloc()
> > and ubc_release() (ala segmap_getmap() and segmap_release()).
> > you get a mapping onto the part of the file the user wants to change,
> > do a uiomove() to copy the user's buffer in, and then release the mapping.
> > mappings are cached in an LRU fashion.
> Why copy and not map, mapping eliminates a copy and conserves memory ???

Mapping is only a win _sometimes_, on _some processors_.  Not all MMUs are
"fast"; many I/O's are "too small".

Heck, why not just have uiomove() map instead of copy?  Clearly it's not
always appropriate.