Messages in this thread | | | From | Tom Zanussi <> | Date | Tue, 12 Jul 2005 10:26:33 -0500 | Subject | Re: Merging relayfs? |
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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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