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relayfs: An efficient unified approach for trasmitting data from kernel to user space
Linux currently has several mechanisms for relaying information
about the system and applications to the user. Some examples
include printk and other syslog events, evlog, ltt, oprofile, etc. Each
subsystem has its own method for relaying information from the
kernel to user space. Some of these mechanisms have difficulties,
e.g., printks get lost. In addition to selected difficulties, the replication
of code and maintenance is undesirable. In this paper we describe
relayfs, a high-speed data relay filesystem, that addresses the needs
of the above subsystems while providing a unified, efficient, and
reliable relay mechanism. relayfs allows subsystems to log data
efficiently and safely using lockless technology that is designed to
scale well across multiprocessor systems. relayfs includes the
flexibility to be expanded should other subsystems need additional
services, but has a simple design intended to meet the needs of
currently available subsystems. In this paper we discuss the
architecture, implementation, and usage of relayfs. relayfs uses
channels that allow data to be directed to a suitable buffer or buffers
for the subsystems that allocated the channel. We describe the
kernel API and file naming conventions, address init-time issues, and
discuss performance tradeoffs available using relayfs. Finally, via
example, we demonstrate how existing subsystems use relayfs to
log their data.
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