232 lines
10 KiB
Plaintext
232 lines
10 KiB
Plaintext
Glock internal locking rules
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------------------------------
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This documents the basic principles of the glock state machine
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internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h)
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has two main (internal) locks:
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1. A spinlock (gl_spin) which protects the internal state such
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as gl_state, gl_target and the list of holders (gl_holders)
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2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other
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threads from making calls to the DLM, etc. at the same time. If a
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thread takes this lock, it must then call run_queue (usually via the
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workqueue) when it releases it in order to ensure any pending tasks
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are completed.
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The gl_holders list contains all the queued lock requests (not
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just the holders) associated with the glock. If there are any
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held locks, then they will be contiguous entries at the head
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of the list. Locks are granted in strictly the order that they
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are queued, except for those marked LM_FLAG_PRIORITY which are
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used only during recovery, and even then only for journal locks.
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There are three lock states that users of the glock layer can request,
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namely shared (SH), deferred (DF) and exclusive (EX). Those translate
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to the following DLM lock modes:
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Glock mode | DLM lock mode
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------------------------------
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UN | IV/NL Unlocked (no DLM lock associated with glock) or NL
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SH | PR (Protected read)
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DF | CW (Concurrent write)
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EX | EX (Exclusive)
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Thus DF is basically a shared mode which is incompatible with the "normal"
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shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O
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operations. The glocks are basically a lock plus some routines which deal
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with cache management. The following rules apply for the cache:
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Glock mode | Cache data | Cache Metadata | Dirty Data | Dirty Metadata
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--------------------------------------------------------------------------
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UN | No | No | No | No
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SH | Yes | Yes | No | No
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DF | No | Yes | No | No
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EX | Yes | Yes | Yes | Yes
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These rules are implemented using the various glock operations which
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are defined for each type of glock. Not all types of glocks use
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all the modes. Only inode glocks use the DF mode for example.
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Table of glock operations and per type constants:
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Field | Purpose
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----------------------------------------------------------------------------
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go_xmote_th | Called before remote state change (e.g. to sync dirty data)
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go_xmote_bh | Called after remote state change (e.g. to refill cache)
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go_inval | Called if remote state change requires invalidating the cache
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go_demote_ok | Returns boolean value of whether its ok to demote a glock
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| (e.g. checks timeout, and that there is no cached data)
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go_lock | Called for the first local holder of a lock
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go_unlock | Called on the final local unlock of a lock
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go_dump | Called to print content of object for debugfs file, or on
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| error to dump glock to the log.
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go_type | The type of the glock, LM_TYPE_.....
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go_callback | Called if the DLM sends a callback to drop this lock
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go_flags | GLOF_ASPACE is set, if the glock has an address space
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| associated with it
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The minimum hold time for each lock is the time after a remote lock
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grant for which we ignore remote demote requests. This is in order to
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prevent a situation where locks are being bounced around the cluster
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from node to node with none of the nodes making any progress. This
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tends to show up most with shared mmaped files which are being written
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to by multiple nodes. By delaying the demotion in response to a
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remote callback, that gives the userspace program time to make
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some progress before the pages are unmapped.
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There is a plan to try and remove the go_lock and go_unlock callbacks
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if possible, in order to try and speed up the fast path though the locking.
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Also, eventually we hope to make the glock "EX" mode locally shared
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such that any local locking will be done with the i_mutex as required
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rather than via the glock.
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Locking rules for glock operations:
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Operation | GLF_LOCK bit lock held | gl_spin spinlock held
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-----------------------------------------------------------------
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go_xmote_th | Yes | No
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go_xmote_bh | Yes | No
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go_inval | Yes | No
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go_demote_ok | Sometimes | Yes
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go_lock | Yes | No
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go_unlock | Yes | No
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go_dump | Sometimes | Yes
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go_callback | Sometimes (N/A) | Yes
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N.B. Operations must not drop either the bit lock or the spinlock
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if its held on entry. go_dump and do_demote_ok must never block.
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Note that go_dump will only be called if the glock's state
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indicates that it is caching uptodate data.
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Glock locking order within GFS2:
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1. i_mutex (if required)
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2. Rename glock (for rename only)
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3. Inode glock(s)
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(Parents before children, inodes at "same level" with same parent in
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lock number order)
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4. Rgrp glock(s) (for (de)allocation operations)
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5. Transaction glock (via gfs2_trans_begin) for non-read operations
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6. Page lock (always last, very important!)
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There are two glocks per inode. One deals with access to the inode
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itself (locking order as above), and the other, known as the iopen
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glock is used in conjunction with the i_nlink field in the inode to
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determine the lifetime of the inode in question. Locking of inodes
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is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
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In general we prefer to lock local locks prior to cluster locks.
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Glock Statistics
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------------------
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The stats are divided into two sets: those relating to the
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super block and those relating to an individual glock. The
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super block stats are done on a per cpu basis in order to
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try and reduce the overhead of gathering them. They are also
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further divided by glock type. All timings are in nanoseconds.
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In the case of both the super block and glock statistics,
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the same information is gathered in each case. The super
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block timing statistics are used to provide default values for
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the glock timing statistics, so that newly created glocks
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should have, as far as possible, a sensible starting point.
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The per-glock counters are initialised to zero when the
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glock is created. The per-glock statistics are lost when
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the glock is ejected from memory.
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The statistics are divided into three pairs of mean and
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variance, plus two counters. The mean/variance pairs are
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smoothed exponential estimates and the algorithm used is
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one which will be very familiar to those used to calculation
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of round trip times in network code. See "TCP/IP Illustrated,
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Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement",
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p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards.
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Unlike the TCP/IP Illustrated case, the mean and variance are
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not scaled, but are in units of integer nanoseconds.
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The three pairs of mean/variance measure the following
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things:
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1. DLM lock time (non-blocking requests)
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2. DLM lock time (blocking requests)
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3. Inter-request time (again to the DLM)
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A non-blocking request is one which will complete right
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away, whatever the state of the DLM lock in question. That
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currently means any requests when (a) the current state of
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the lock is exclusive, i.e. a lock demotion (b) the requested
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state is either null or unlocked (again, a demotion) or (c) the
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"try lock" flag is set. A blocking request covers all the other
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lock requests.
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There are two counters. The first is there primarily to show
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how many lock requests have been made, and thus how much data
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has gone into the mean/variance calculations. The other counter
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is counting queuing of holders at the top layer of the glock
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code. Hopefully that number will be a lot larger than the number
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of dlm lock requests issued.
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So why gather these statistics? There are several reasons
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we'd like to get a better idea of these timings:
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1. To be able to better set the glock "min hold time"
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2. To spot performance issues more easily
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3. To improve the algorithm for selecting resource groups for
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allocation (to base it on lock wait time, rather than blindly
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using a "try lock")
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Due to the smoothing action of the updates, a step change in
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some input quantity being sampled will only fully be taken
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into account after 8 samples (or 4 for the variance) and this
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needs to be carefully considered when interpreting the
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results.
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Knowing both the time it takes a lock request to complete and
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the average time between lock requests for a glock means we
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can compute the total percentage of the time for which the
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node is able to use a glock vs. time that the rest of the
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cluster has its share. That will be very useful when setting
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the lock min hold time.
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Great care has been taken to ensure that we
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measure exactly the quantities that we want, as accurately
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as possible. There are always inaccuracies in any
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measuring system, but I hope this is as accurate as we
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can reasonably make it.
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Per sb stats can be found here:
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/sys/kernel/debug/gfs2/<fsname>/sbstats
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Per glock stats can be found here:
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/sys/kernel/debug/gfs2/<fsname>/glstats
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Assuming that debugfs is mounted on /sys/kernel/debug and also
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that <fsname> is replaced with the name of the gfs2 filesystem
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in question.
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The abbreviations used in the output as are follows:
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srtt - Smoothed round trip time for non-blocking dlm requests
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srttvar - Variance estimate for srtt
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srttb - Smoothed round trip time for (potentially) blocking dlm requests
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srttvarb - Variance estimate for srttb
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sirt - Smoothed inter-request time (for dlm requests)
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sirtvar - Variance estimate for sirt
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dlm - Number of dlm requests made (dcnt in glstats file)
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queue - Number of glock requests queued (qcnt in glstats file)
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The sbstats file contains a set of these stats for each glock type (so 8 lines
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for each type) and for each cpu (one column per cpu). The glstats file contains
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a set of these stats for each glock in a similar format to the glocks file, but
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using the format mean/variance for each of the timing stats.
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The gfs2_glock_lock_time tracepoint prints out the current values of the stats
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for the glock in question, along with some addition information on each dlm
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reply that is received:
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status - The status of the dlm request
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flags - The dlm request flags
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tdiff - The time taken by this specific request
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(remaining fields as per above list)
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