Re: Merging relayfs?

[Tom Zanussi <zanussi@us.ibm.com>]

Jason Baron writes:
 > 
 > On Mon, 11 Jul 2005, Tom Zanussi wrote:
 > 
 > > 
 > > Hi Andrew, can you please merge relayfs?  It provides a low-overhead
 > > logging and buffering capability, which does not currently exist in
 > > the kernel.
 > > 
 > 
 > One concern I had regarding relayfs, which was raised previously, was 
 > regarding its use of vmap, 
 > http://marc.theaimsgroup.com/?l=linux-kernel&m=110755199913216&w=2 On x86, 
 > the vmap space is at a premium, and this space is reserved over the entire 
 > lifetime of a 'channel'. Is the use of vmap really critical for 
 > performance?

Yes, the vmap'ed area is reserved over the lifetime of the channel,
but the typical usage of a channel is transient - allocate it at the
start of say a tracing run, and then vunmap it and free the memory
when done.  Unless you're using huge buffers, you wouldn't run into a
problem running out of vmalloc space, and typical applications should
be able to use relatively small buffers.

I don't really know how we would get around using vmap - it seems like
the alternatives, such as managing an array of pages or something like
that, would slow down the logging path too much to make it useful as a
low overhead logging mechanism.  I you have any ideas though, please
let me know.

Tom