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Re: subversion _really_ slow

From: Pete Gonzalez <pgonzalez_at_bluel.com>
Date: 2005-02-15 18:39:29 CET

> > I currently have a problem with generating random numbers (/dev/random
> takes
> > undefinied amount of time) which slows things very much.
> > That could explain the 'unpredictable' factor: maybe you don't have enough
> > random numbers on your system. But do a strace and it'll help understand
> > where the process 'hangs'.
>Oh, yeah, this happened to me too!
>I recompiled APR with "--with-devrandom=/dev/urandom", then recompiled
>my Subversion client against that APR, and the problem went away.

I have thought about this problem, and I would like to argue that
"/dev/urandom" (or even better the alternate timer-based implementation)
should be the *default*, rather than being a retarded "gotcha" for
new users:

This code is called from APR's getuuid.c (which generates the UUID's),
and the UUID spec referenced there does not require the numbers to be
truly random.

In my case, the /dev/random issue turned out to be a design problem
in the 2.4 Linux kernel (see below). I've also seen several newsgroup
discussions discussing improper default configurations for /dev/random,
so this situation can occur in several ways.

Although some people will figure it out and begrudgingly recompile
their server, I think most admins will just conclude that Subversion
is inefficient by design. Which, I argue, is not far from the truth
if /dev/random is kept as the default. ;-)


------------Forwarded Message-----------
Date: Tue, 15 Feb 2005 10:59:38 -0500
From: Theodore Ts'o <tytso@mit.edu>
To: Pete Gonzalez <pgonzalez@bluel.com>
Subject: Re: Question regarding /dev/urandom design

> I'm writing with a question regarding your random.c driver for the
> Linux kernel. It would appear that the intent of "/dev/urandom"
> is to supply random numbers for applications that need immediate
> results, and where quality can be sacrificed for quantity. However,
> it appears that when an application reads from /dev/urandom
> frequently, other applications reading from /dev/random will hang
> indefinitely because the entropy is being constantly depleted.
> If so, then IMO this defeats the purpose of "/dev/urandom", since
> although it offers an unlimited resource for the calling application,
> it is still a limited resource from the OS's perspective. A possible
> solution would be to maintain a separate entropy pool for
> "/dev/urandom", or maybe somehow the "/dev/random" requests can
> be prioritized over "/dev/urandom".

It is a separate entropy pool in 2.6, but people still use it wrong,
because they seem to use it instead of a cryptographic random
generator, which is what most of them *really* seem to want. All you
should do is read 16 bytes from /dev/random, and use it to seed a SHA
based random number generator --- which you use in userspace! It's
faster, and really the right answer.

                                                 - Ted

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Received on Tue Feb 15 18:59:27 2005

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