Re: Relayfs question

[Karim Yaghmour <karim@opersys.com>]

Jan Engelhardt wrote:
> Well, what about things like urandom? It also moves "a lot" of data and does
> nothing else.

Forgive my slowness today, but I don't get the angle here:
- Relayfs is not a replacement for char devices, we've never claimed it
  to be.
- Urandom generates a lot of data, and uses copy_to_user() to get it to
  user-space, but it isn't a generalized buffering mechanism for
  transfering large amounts of data to user-space.

If what you're inquiring about is a comparison between relayfs'
mechanisms and the underlying mechanisms that urandom is using, then
I don't think there can be a comparison: the goals are different.

For example, urandom relies on a global spin lock and uses copy_to_user()
for its transfers. This is just fine for this type of application. If
you wanted to transfer a huge amount of data from the kernel to user-
space (the kind of data generated by tracing facilities, for example),
however, these mechanisms would be simply inadequate. If we're generating
the amount of data LTT can gather, for example, (say 2MB/s as was
described in the earlier thread regarding relayfs), then you need per-cpu
buffering and you need to not write anything back to user-space, but
dump it to disk ASAP, etc. This is where relayfs comes in handy.

On the other hand, using relayfs to replace what urandom currently uses
is just the wrong thing to do. If nothing else, /dev/urandom would
behave entirely differently (API, dynamics, etc.). There would also be
no clear added benefit for using relayfs.

What character drivers do (mainly copy_to_user()) and what relayfs is
used for are entirely different. To use a slightly exagerated example
to illustrate the difference: replacing the standard mechanisms drivers
use to transfer data to user-space with relayfs would be like renting
a supersonic jet to get your package to a foreign country instead of
just using Fedex. It works ... but it's clearly the wrong approach.

Please read relayfs.txt.

Karim
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