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Subjectptrace performance (was: [Bug #12208] uml is very slow on 2.6.28 host)
Disclaimer: I'm not using UML, but these problems may be related.

> Bug-Entry :
> Subject : uml is very slow on 2.6.28 host
> Submitter : Miklos Szeredi <>
> Date : 2008-12-12 9:35 (93 days old)
> References :

The other day I noticed a dramatic ptrace slowdown between 2.6.27 and
2.6.28.x (verified with 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 <>
X-Tired: Each morning I get up I die a little

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