freebsd/sys/geom/sched

	--- GEOM BASED DISK SCHEDULERS FOR FREEBSD ---

This code contains a framework for GEOM-based disk schedulers and a
couple of sample scheduling algorithms that use the framework and
implement two forms of "anticipatory scheduling" (see below for more
details).

As a quick example of what this code can give you, try to run "dd",
"tar", or some other program with highly SEQUENTIAL access patterns,
together with "cvs", "cvsup", "svn" or other highly RANDOM access patterns
(this is not a made-up example: it is pretty common for developers
to have one or more apps doing random accesses, and others that do
sequential accesses e.g., loading large binaries from disk, checking
the integrity of tarballs, watching media streams and so on).

These are the results we get on a local machine (AMD BE2400 dual
core CPU, SATA 250GB disk):

    /mnt is a partition mounted on /dev/ad0s1f

    cvs: 	cvs -d /mnt/home/ncvs-local update -Pd /mnt/ports
    dd-read:	dd bs=128k of=/dev/null if=/dev/ad0 (or ad0-sched-)
    dd-writew	dd bs=128k if=/dev/zero of=/mnt/largefile

			NO SCHEDULER		RR SCHEDULER
                	dd	cvs		dd	cvs

    dd-read only        72 MB/s	----		72 MB/s	---
    dd-write only	55 MB/s	---		55 MB/s	---
    dd-read+cvs		 6 MB/s	ok    		30 MB/s	ok
    dd-write+cvs	55 MB/s slooow		14 MB/s	ok

As you can see, when a cvs is running concurrently with dd, the
performance drops dramatically, and depending on read or write mode,
one of the two is severely penalized.  The use of the RR scheduler
in this example makes the dd-reader go much faster when competing
with cvs, and lets cvs progress when competing with a writer.

To try it out:

1. PLEASE MAKE SURE THAT THE DISK THAT YOU WILL BE USING FOR TESTS
   DOES NOT CONTAIN PRECIOUS DATA.
    This is experimental code, so we make no guarantees, though
    I am routinely using it on my desktop and laptop.

2. EXTRACT AND BUILD THE PROGRAMS
    A 'make install' in the directory should work (with root privs),
    or you can even try the binary modules.
    If you want to build the modules yourself, look at the Makefile.

3. LOAD THE MODULE, CREATE A GEOM NODE, RUN TESTS

    The scheduler's module must be loaded first:

      # kldload gsched_rr

    substitute with gsched_as to test AS.  Then, supposing that you are
    using /dev/ad0 for testing, a scheduler can be attached to it with:

      # geom sched insert ad0

    The scheduler is inserted transparently in the geom chain, so
    mounted partitions and filesystems will keep working, but
    now requests will go through the scheduler.

    To change scheduler on-the-fly, you can reconfigure the geom:

      # geom sched configure -a as ad0.sched.

    assuming that gsched_as was loaded previously.

5. SCHEDULER REMOVAL

    In principle it is possible to remove the scheduler module
    even on an active chain by doing

	# geom sched destroy ad0.sched.

    However, there is some race in the geom subsystem which makes
    the removal unsafe if there are active requests on a chain.
    So, in order to reduce the risk of data losses, make sure
    you don't remove a scheduler from a chain with ongoing transactions.

--- NOTES ON THE SCHEDULERS ---

The important contribution of this code is the framework to experiment
with different scheduling algorithms.  'Anticipatory scheduling'
is a very powerful technique based on the following reasoning:

    The disk throughput is much better if it serves sequential requests.
    If we have a mix of sequential and random requests, and we see a
    non-sequential request, do not serve it immediately but instead wait
    a little bit (2..5ms) to see if there is another one coming that
    the disk can serve more efficiently.

There are many details that should be added to make sure that the
mechanism is effective with different workloads and systems, to
gain a few extra percent in performance, to improve fairness,
insulation among processes etc.  A discussion of the vast literature
on the subject is beyond the purpose of this short note.

--------------------------------------------------------------------------

TRANSPARENT INSERT/DELETE

geom_sched is an ordinary geom module, however it is convenient
to plug it transparently into the geom graph, so that one can
enable or disable scheduling on a mounted filesystem, and the
names in /etc/fstab do not depend on the presence of the scheduler.

To understand how this works in practice, remember that in GEOM
we have "providers" and "geom" objects.
Say that we want to hook a scheduler on provider "ad0",
accessible through pointer 'pp'. Originally, pp is attached to
geom "ad0" (same name, different object) accessible through pointer old_gp

  BEFORE	---> [ pp    --> old_gp ...]

A normal "geom sched create ad0" call would create a new geom node
on top of provider ad0/pp, and export a newly created provider
("ad0.sched." accessible through pointer newpp).

  AFTER create  ---> [ newpp --> gp --> cp ] ---> [ pp    --> old_gp ... ]

On top of newpp, a whole tree will be created automatically, and we
can e.g. mount partitions on /dev/ad0.sched.s1d, and those requests
will go through the scheduler, whereas any partition mounted on
the pre-existing device entries will not go through the scheduler.

With the transparent insert mechanism, the original provider "ad0"/pp
is hooked to the newly created geom, as follows:

  AFTER insert  ---> [ pp    --> gp --> cp ] ---> [ newpp --> old_gp ... ]

so anything that was previously using provider pp will now have
the requests routed through the scheduler node.

A removal ("geom sched destroy ad0.sched.") will restore the original
configuration.

# $FreeBSD$