/dev/zero performance drop between SLE10 SP1 and SP2

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Environment

SUSE Linux Enterprise Server 10 SP2
SUSE Linux Enterprise Desktop 10 SP2

Situation

Changes

A SUSE Linux Enterprise 10 system was updated from Support Pack 1 to Support Pack 2.

Symptoms

The read/write performance of /dev/zero as measured e.g. by
time dd if=/dev/zero of=/dev/null bs=1G count=1
has dropped significantly.
I/O performance as reported by benchmark software has dropped significantly.

Resolution

This issue generally does not affect real life workloads, only benchmarks which rely on a fast /dev/zero. SUSE Engineering is researching ways of restoring /dev/zero's performance without reintroducing the problems associated with the implementation that was used prior to Service Pack 2.

Additional Information

Background

/dev/zero is a special character device file. When data is written to it, it is discarded, i.e. it operates as a "bit bucket". When data is read from it, it produces a stream of bytes of value zero.

Prior to Service Pack 2, the kernel had a special mechanism, called "ZERO_PAGE", which was used to implement /dev/zero. In Service Pack 2's kernel, this mechanism has been removed as on larger systems, it could cause livelocks (a form of resource starvation). The reduced performance of /dev/zero is a side effect of this removal.

Disclaimer

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