*
* CDDL HEADER END
*/
+
/*
* Copyright 2009 Sun Microsystems, Inc. All rights reserved.
* Use is subject to license terms.
*/
+/*
+ * Copyright (c) 2013 by Delphix. All rights reserved.
+ */
+
#ifndef _SYS_TXG_IMPL_H
#define _SYS_TXG_IMPL_H
extern "C" {
#endif
+/*
+ * The tx_cpu structure is a per-cpu structure that is used to track
+ * the number of active transaction holds (tc_count). As transactions
+ * are assigned into a transaction group the appropriate tc_count is
+ * incremented to indicate that there are pending changes that have yet
+ * to quiesce. Consumers evenutally call txg_rele_to_sync() to decrement
+ * the tc_count. A transaction group is not considered quiesced until all
+ * tx_cpu structures have reached a tc_count of zero.
+ *
+ * This structure is a per-cpu structure by design. Updates to this structure
+ * are frequent and concurrent. Having a single structure would result in
+ * heavy lock contention so a per-cpu design was implemented. With the fanned
+ * out mutex design, consumers only need to lock the mutex associated with
+ * thread's cpu.
+ *
+ * The tx_cpu contains two locks, the tc_lock and tc_open_lock.
+ * The tc_lock is used to protect all members of the tx_cpu structure with
+ * the exception of the tc_open_lock. This lock should only be held for a
+ * short period of time, typically when updating the value of tc_count.
+ *
+ * The tc_open_lock protects the tx_open_txg member of the tx_state structure.
+ * This lock is used to ensure that transactions are only assigned into
+ * the current open transaction group. In order to move the current open
+ * transaction group to the quiesce phase, the txg_quiesce thread must
+ * grab all tc_open_locks, increment the tx_open_txg, and drop the locks.
+ * The tc_open_lock is held until the transaction is assigned into the
+ * transaction group. Typically, this is a short operation but if throttling
+ * is occurring it may be held for longer periods of time.
+ */
struct tx_cpu {
- kmutex_t tc_lock;
+ kmutex_t tc_open_lock; /* protects tx_open_txg */
+ kmutex_t tc_lock; /* protects the rest of this struct */
kcondvar_t tc_cv[TXG_SIZE];
- uint64_t tc_count[TXG_SIZE];
+ uint64_t tc_count[TXG_SIZE]; /* tx hold count on each txg */
list_t tc_callbacks[TXG_SIZE]; /* commit cb list */
- char tc_pad[16];
+ char tc_pad[8]; /* pad to fill 3 cache lines */
};
+/*
+ * The tx_state structure maintains the state information about the different
+ * stages of the pool's transcation groups. A per pool tx_state structure
+ * is used to track this information. The tx_state structure also points to
+ * an array of tx_cpu structures (described above). Although the tx_sync_lock
+ * is used to protect the members of this structure, it is not used to
+ * protect the tx_open_txg. Instead a special lock in the tx_cpu structure
+ * is used. Readers of tx_open_txg must grab the per-cpu tc_open_lock.
+ * Any thread wishing to update tx_open_txg must grab the tc_open_lock on
+ * every cpu (see txg_quiesce()).
+ */
typedef struct tx_state {
- tx_cpu_t *tx_cpu; /* protects right to enter txg */
- kmutex_t tx_sync_lock; /* protects tx_state_t */
+ tx_cpu_t *tx_cpu; /* protects access to tx_open_txg */
+ kmutex_t tx_sync_lock; /* protects the rest of this struct */
+
uint64_t tx_open_txg; /* currently open txg id */
uint64_t tx_quiesced_txg; /* quiesced txg waiting for sync */
uint64_t tx_syncing_txg; /* currently syncing txg id */
uint64_t tx_synced_txg; /* last synced txg id */
+ hrtime_t tx_open_time; /* start time of tx_open_txg */
+
uint64_t tx_sync_txg_waiting; /* txg we're waiting to sync */
uint64_t tx_quiesce_txg_waiting; /* txg we're waiting to open */
projects / zfs / blobdiff