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GFS2: Update glock doc to add new stats info

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We recently added some glock statistics to GFS2, so this is
a docs update to explain what they all mean. It is based
upon the checkin comment of the patch in question.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
This commit is contained in:
Steven Whitehouse 2012-05-10 12:41:40 +01:00
parent 49f30789fc
commit 2ebc3f8b3e
1 changed files with 118 additions and 1 deletions
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119
Documentation/filesystems/gfs2-glocks.txt
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@ -61,7 +61,9 @@ go_unlock | Called on the final local unlock of a lock
go_dump | Called to print content of object for debugfs file, or on
| error to dump glock to the log.
go_type | The type of the glock, LM_TYPE_.....
go_min_hold_time | The minimum hold time
go_callback | Called if the DLM sends a callback to drop this lock
go_flags | GLOF_ASPACE is set, if the glock has an address space
| associated with it
The minimum hold time for each lock is the time after a remote lock
grant for which we ignore remote demote requests. This is in order to
@ -89,6 +91,7 @@ go_demote_ok | Sometimes | Yes
go_lock | Yes | No
go_unlock | Yes | No
go_dump | Sometimes | Yes
go_callback | Sometimes (N/A) | Yes
N.B. Operations must not drop either the bit lock or the spinlock
if its held on entry. go_dump and do_demote_ok must never block.
@ -111,4 +114,118 @@ itself (locking order as above), and the other, known as the iopen
glock is used in conjunction with the i_nlink field in the inode to
determine the lifetime of the inode in question. Locking of inodes
is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
In general we prefer to lock local locks prior to cluster locks.
Glock Statistics
------------------
The stats are divided into two sets: those relating to the
super block and those relating to an individual glock. The
super block stats are done on a per cpu basis in order to
try and reduce the overhead of gathering them. They are also
further divided by glock type. All timings are in nanoseconds.
In the case of both the super block and glock statistics,
the same information is gathered in each case. The super
block timing statistics are used to provide default values for
the glock timing statistics, so that newly created glocks
should have, as far as possible, a sensible starting point.
The per-glock counters are initialised to zero when the
glock is created. The per-glock statistics are lost when
the glock is ejected from memory.
The statistics are divided into three pairs of mean and
variance, plus two counters. The mean/variance pairs are
smoothed exponential estimates and the algorithm used is
one which will be very familiar to those used to calculation
of round trip times in network code. See "TCP/IP Illustrated,
Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement",
p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards.
Unlike the TCP/IP Illustrated case, the mean and variance are
not scaled, but are in units of integer nanoseconds.
The three pairs of mean/variance measure the following
things:
1. DLM lock time (non-blocking requests)
2. DLM lock time (blocking requests)
3. Inter-request time (again to the DLM)
A non-blocking request is one which will complete right
away, whatever the state of the DLM lock in question. That
currently means any requests when (a) the current state of
the lock is exclusive, i.e. a lock demotion (b) the requested
state is either null or unlocked (again, a demotion) or (c) the
"try lock" flag is set. A blocking request covers all the other
lock requests.
There are two counters. The first is there primarily to show
how many lock requests have been made, and thus how much data
has gone into the mean/variance calculations. The other counter
is counting queuing of holders at the top layer of the glock
code. Hopefully that number will be a lot larger than the number
of dlm lock requests issued.
So why gather these statistics? There are several reasons
we'd like to get a better idea of these timings:
1. To be able to better set the glock "min hold time"
2. To spot performance issues more easily
3. To improve the algorithm for selecting resource groups for
allocation (to base it on lock wait time, rather than blindly
using a "try lock")
Due to the smoothing action of the updates, a step change in
some input quantity being sampled will only fully be taken
into account after 8 samples (or 4 for the variance) and this
needs to be carefully considered when interpreting the
results.
Knowing both the time it takes a lock request to complete and
the average time between lock requests for a glock means we
can compute the total percentage of the time for which the
node is able to use a glock vs. time that the rest of the
cluster has its share. That will be very useful when setting
the lock min hold time.
Great care has been taken to ensure that we
measure exactly the quantities that we want, as accurately
as possible. There are always inaccuracies in any
measuring system, but I hope this is as accurate as we
can reasonably make it.
Per sb stats can be found here:
/sys/kernel/debug/gfs2/<fsname>/sbstats
Per glock stats can be found here:
/sys/kernel/debug/gfs2/<fsname>/glstats
Assuming that debugfs is mounted on /sys/kernel/debug and also
that <fsname> is replaced with the name of the gfs2 filesystem
in question.
The abbreviations used in the output as are follows:
srtt - Smoothed round trip time for non-blocking dlm requests
srttvar - Variance estimate for srtt
srttb - Smoothed round trip time for (potentially) blocking dlm requests
srttvarb - Variance estimate for srttb
sirt - Smoothed inter-request time (for dlm requests)
sirtvar - Variance estimate for sirt
dlm - Number of dlm requests made (dcnt in glstats file)
queue - Number of glock requests queued (qcnt in glstats file)
The sbstats file contains a set of these stats for each glock type (so 8 lines
for each type) and for each cpu (one column per cpu). The glstats file contains
a set of these stats for each glock in a similar format to the glocks file, but
using the format mean/variance for each of the timing stats.
The gfs2_glock_lock_time tracepoint prints out the current values of the stats
for the glock in question, along with some addition information on each dlm
reply that is received:
status - The status of the dlm request
flags - The dlm request flags
tdiff - The time taken by this specific request
(remaining fields as per above list)