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SubjectRe: Merging relayfs?
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


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