/dev/zero performance drop between SLE10 SP1 and SP2

This document (7002112) is provided subject to the disclaimer at the end of this document.

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

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

  • Document ID:7002112
  • Creation Date: 08-Dec-2008
  • Modified Date:10-Mar-2021
    • SUSE Linux Enterprise Desktop
    • SUSE Linux Enterprise Server

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