Re: [Qemu-devel] [PATCH RFC 1/2] rng-egd: improve egd backend performance

[Amos Kong]

On Wed, Jan 08, 2014 at 09:53:02PM +0530, Amit Shah wrote:
> On (Wed) 08 Jan 2014 [17:14:41], Amos Kong wrote:
> > On Wed, Dec 18, 2013 at 11:05:14AM +0100, Giuseppe Scrivano wrote:
> > > Markus Armbruster <address@hidden> writes:
> > > 
> > > > Amos Kong <address@hidden> writes:
> > > >
> > > >> Bugzilla: https://bugs.launchpad.net/qemu/+bug/1253563
> > > >>
> > > >> We have a requests queue to cache the random data, but the second
> > > >> will come in when the first request is returned, so we always
> > > >> only have one items in the queue. It effects the performance.
> > > >>
> > > >> This patch changes the IOthread to fill a fixed buffer with
> > > >> random data from egd socket, request_entropy() will return
> > > >> data to virtio queue if buffer has available data.
> > > >>
> > > >> (test with a fast source, disguised egd socket)
> > > >>  # cat /dev/urandom | nc -l localhost 8003
> > > >>  # qemu .. -chardev socket,host=localhost,port=8003,id=chr0 \
> > > >>         -object rng-egd,chardev=chr0,id=rng0,buf_size=1024 \
> > > >>         -device virtio-rng-pci,rng=rng0
> > > >>
> > > >>   bytes     kb/s
> > > >>   ------    ----
> > > >>   131072 ->  835
> > > >>    65536 ->  652
> > > >>    32768 ->  356
> > > >>    16384 ->  182
> > > >>     8192 ->   99
> > > >>     4096 ->   52
> > > >>     2048 ->   30
> > > >>     1024 ->   15
> > > >>      512 ->    8
> > > >>      256 ->    4
> > > >>      128 ->    3
> > > >>       64 ->    2
> > > >
> > > > I'm not familiar with the rng-egd code, but perhaps my question has
> > > > value anyway: could agressive reading ahead on a source of randomness
> > > > cause trouble by depleting the source?
> > > >
> > > > Consider a server restarting a few dozen guests after reboot, where each
> > > > guest's QEMU then tries to slurp in a couple of KiB of randomness.  How
> > > > does this behave?
> > 
> > Hi Giuseppe,
> >  
> > > I hit this performance problem while I was working on RNG devices
> > > support in virt-manager and I also noticed that the bottleneck is in the
> > > egd backend that slowly response to requests.
> > 
> > o Current situation:
> >   rng-random backend reads data from non-blocking character devices
> >   New entropy request will be sent from guest when last request is 
> > processed,
> >   so the request queue can only cache one request.
> >   Almost all the request size is 64 bytes.
> >   Egd socket responses the request slowly.
> > 
> > o Solution 1: pre-reading, perf is improved, but cost much memory 
> >   In my V1 patch, I tried to add a configurable buffer to pre-read data
> >   from egd socket. The performance was improved but it used a big memory
> >   as the buffer.
> 
> I really dislike buffering random numbers or entropy from the host,
> let's rule these options out.

Agree.
The main reason is the slow source, using buffers and pre-reading is
just abuse the resource & sync read API in qemu.
 
> > o Solution 2: pre-sending request to egd socket, improve is trivial
> >   I did another test, we just pre-send entropy request to egd socket, not
> >   really read the data to a buffer.
> > 
> > o Solution 3: eyeless poll, not good
> >   Always returns an integer in rng_egd_chr_can_read(), the perf can be 
> >   improved to 120 kB/s, it reduce the delay caused by poll mechanism.
> > 
> > o Solution 4:
> >   Try to use the new message type to improve the response speed of egd 
> > socket
> > 
> > o Solution 5:
> >   non-block read?
> 
> I'd just say let the "problem" be.  I don't really get the point of
> egd.  The egd backend was something Anthony wanted, but I can't
> remember if there has been enough justification for it.  Certainly the
> protocol isn't documented, and not using the backend doesn't give us
> drawbacks.
 
http://miketeo.net/wp/index.php/2009/06/09/egd-entropy-gathering-daemon-client-protocol.html

> Moreover, reasonable guests won't request for a whole lot of random
> numbers in a short interval, so the theoretical performance problem
> we're seeing is just going to remain theoretical for well-behaved
> guests.
> 
> We have enough documentation by now about this issue, I say let's just
> drop this patch and worry about this only if there's a proven need to
> better things here.

We always recommend the users to use rng-random backend. Only use
rng-egd backend when host uses a USB entropy device.

Let's see if there exists some problem when the rng backend speed is
about 5 kB/s.
 
>               Amit

-- 
                        Amos.