ptrace performance (was: [Bug #12208] uml is very slow on 2.6.28host)

[Michael Riepe]

Disclaimer: I'm not using UML, but these problems may be related.

> Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=12208
> Subject		: uml is very slow on 2.6.28 host
> Submitter	: Miklos Szeredi <miklos@xxxxxxxxxx>
> Date		: 2008-12-12 9:35 (93 days old)
> References	: http://marc.info/?l=linux-kernel&m=122907463518593&w=4

The other day I noticed a dramatic ptrace slowdown between 2.6.27 and
2.6.28.x (verified with 2.6.28.8). In particular, a command like

	dd if=/dev/zero of=/dev/null bs=1024k count=1024

will normally report a throughput in the GB/s range. On 2.6.27, this is
also true if you run

	strace -o /dev/null <dd command as above>

which is only a little slower. But if I do the same on 2.6.28.x, I get a
throughput of about 100 MB/s or less, i.e. less than 10%. I tried the
commands on three different machines (an Athlon64 3000+, a Core Duo
T2400 and an Atom 330), and they all behave similar. The more system
calls a program uses, the worse the slowdown (try the dd command with
bs=16k and count=65536, for example - but don't hold your breath).

Interestingly, the CPUs are mostly idle while the command is executing
on 2.6.28.x, but there is a high (system) load on 2.6.27. Therefore, I
suspect that it's a scheduling or maybe timer problem that was
introduced between 2.6.27 and 2.6.28. I haven't had the time to check
the rc kernels yet; perhaps someone else can run a quick check to verify
that it's gone in the latest 2.6.29-rc.

-- 
Michael "Tired" Riepe <michael.riepe@xxxxxxxxxxxxxx>
X-Tired: Each morning I get up I die a little
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