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Subjectdirty_expire_centisecs, msync behavior
The documentation for dirty_expire_centisecs states: "Data which has been 
dirty in-memory for longer than this interval will be written out next time a
flusher thread wakes up."

In practice, it appears that once the expire time has passed, all dirty pages
get flushed, regardless of their age. This behavior makes this setting fairly
useless. This appears to have been the behavior for most of 2.6 and 3.x. Can
anyone explain, is the current behavior really as intended, and is the doc
just out of date?

On a slightly related note, what was the key problem with this patch "msync:
support syncing a small part of the file"?
http://thread.gmane.org/gmane.linux.kernel/1313767/focus=1317498

Andrew Morton's message states that Paolo's patch would break nonlinear
mappings, and the matter was dropped. Why wasn't it possible to write a patch
that would also work with nonlinear mappings? I couldn't find any earlier
context for that subject, pointers welcome.

My interest in both of these questions stems from what I've observed while
testing the LMDB memory-mapped database. On a machine with 32GB RAM, using a
database that occupies about 18GB of memory, doing continuous writes to the DB
without ever calling msync, and default writeback settings, I see DB
throughput spike downward every time the flusher wakes up. The DB is a mmap'd
file on an XFS partition, and a DB write operation simply dirties a random set
of pages. After the program has been running for more than
dirty_expire_centisecs, every dirty_writeback_centisecs the DB app basically
stops while the flusher writes out all the dirty pages.

I'm curious about a couple things - since the DB knows which pages it is
dirtying in a given transaction, would it help overall throughput if the DB
told the OS (via msync) exactly which ranges to flush? Obviously not, in the
current implementation of msync, but can a patch like Paolo's make this
better? And can the dirty_expire_centisecs behavior be fixed, so that it's
only writing out a smaller set of pages on each wakeup? What else can we do to
minimize the impact of the flusher? If I turn it off completely the throughput
nearly doubles, from 5100 DB writes/sec to 9000/sec. If I turn off the timed
flush and just use dirty_background_bytes the throughput just slows to around
7000/sec.

It seems to me the main slowdown is because the OS is locking dirty pages
indiscriminately. The DB does copy-on-write, so pages that it dirties in one
transaction will not be written again in the next transaction. I would have
expected read-only accesses to these pages to be able to progress without any
delay but that doesn't seem to be the case.

--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/


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