diff options
Diffstat (limited to 'include/sys/txg_impl.h')
| -rw-r--r-- | include/sys/txg_impl.h | 49 |
1 files changed, 47 insertions, 2 deletions
diff --git a/include/sys/txg_impl.h b/include/sys/txg_impl.h index 7b356eac1..5a6d0e19f 100644 --- a/include/sys/txg_impl.h +++ b/include/sys/txg_impl.h @@ -23,6 +23,10 @@ * Use is subject to license terms. */ +/* + * Copyright (c) 2013 by Delphix. All rights reserved. + */ + #ifndef _SYS_TXG_IMPL_H #define _SYS_TXG_IMPL_H @@ -33,14 +37,55 @@ 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 occuring 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]; 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 */ |