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authorSerapheim Dimitropoulos <[email protected]>2017-12-05 09:45:46 -0800
committerBrian Behlendorf <[email protected]>2018-06-04 14:56:06 -0700
commite48afbc4ebc64712f56220b65ed12c536ef0e114 (patch)
tree5e4a96202cff3f608937f268398d6189281650cc /man
parentab44e511e2546567a296a88e71ed2056372fd5a4 (diff)
OpenZFS 9464 - txg_kick() fails to see that we are quiescing
txg_kick() fails to see that we are quiescing, forcing transactions to their next stages without leaving them accumulate changes Creating a fragmented pool in a DCenter VM and continuously writing to it with multiple instances of randwritecomp, we get the following output from txg.d: 0ms 311MB in 4114ms (95% p1) 75MB/s 544MB (76%) 336us 153ms 0ms 0ms 8MB in 51ms ( 0% p1) 163MB/s 474MB (66%) 129us 34ms 0ms 0ms 366MB in 4454ms (93% p1) 82MB/s 572MB (79%) 498us 20ms 0ms 0ms 406MB in 5212ms (95% p1) 77MB/s 591MB (82%) 661us 37ms 0ms 0ms 340MB in 5110ms (94% p1) 66MB/s 622MB (86%) 1048us 41ms 1ms 0ms 3MB in 61ms ( 0% p1) 51MB/s 419MB (58%) 33us 0ms 0ms 0ms 361MB in 3555ms (88% p1) 101MB/s 542MB (75%) 335us 40ms 0ms 0ms 356MB in 4592ms (92% p1) 77MB/s 561MB (78%) 430us 89ms 1ms 0ms 11MB in 129ms (13% p1) 90MB/s 507MB (70%) 222us 15ms 0ms 0ms 281MB in 2520ms (89% p1) 111MB/s 542MB (75%) 334us 42ms 0ms 0ms 383MB in 3666ms (91% p1) 104MB/s 557MB (77%) 411us 133ms 0ms 0ms 404MB in 5757ms (94% p1) 70MB/s 635MB (88%) 1274us 123ms 2ms 4ms 367MB in 4172ms (89% p1) 88MB/s 556MB (77%) 401us 51ms 0ms 0ms 42MB in 470ms (44% p1) 90MB/s 557MB (77%) 412us 43ms 0ms 0ms 261MB in 2273ms (88% p1) 114MB/s 556MB (77%) 407us 27ms 0ms 0ms 394MB in 3646ms (85% p1) 108MB/s 552MB (77%) 393us 304ms 0ms 0ms 275MB in 2416ms (89% p1) 113MB/s 510MB (71%) 200us 53ms 0ms 0ms 9MB in 53ms ( 0% p1) 169MB/s 483MB (67%) 140us 100ms 1ms The TXGs that are getting synced and don't have lots of changes are pushed by txg_kick() which basically forces the current open txg to get to the quiesced state: if (tx->tx_syncing_txg == 0 && tx->tx_quiesce_txg_waiting <= tx->tx_open_txg && tx->tx_sync_txg_waiting <= tx->tx_synced_txg && tx->tx_quiesced_txg <= tx->tx_synced_txg) { tx->tx_quiesce_txg_waiting = tx->tx_open_txg + 1; cv_broadcast(&tx->tx_quiesce_more_cv); } The problem is that the above code doesn't check if we are currently quiescing anything (only if a quiesce or a sync has been requested, ..etc) so the following scenario can happen: 1] We have an open txg A that had enough dirty data (more than zfs_dirty_data_sync) and it was pushed to the quiesced state, and opened a new txg B. No txg is currently being synced. 2] Immediately after the opening of B, txg_kick() was run by some other write (and because of A's dirty data) and saw that we are not currently syncing any txg and no one has requested quiescing so it requests one by bumping tx_quiesce_txg_waiting and broadcasts the quiesce thread. 3] The quiesce thread just passed txg A to be synced and sees that a quiescing request has been sent to it so it immediately grabs B without letting it gather enough data, putting it in a quiesced state and opening a new txg C. In this scenario txg B, is an example of how the entries of interest show up in the txg.d output. Ideally we would like txg_kick() to get triggered only when we are sure that we are not syncing AND not quiescing any txg. This way we can kick an open TXG to the quiescing state when we are sure that there is nothing going on and we would benefit from the different states running concurrently. Authored by: Serapheim Dimitropoulos <[email protected]> Reviewed by: Matt Ahrens <[email protected]> Reviewed by: Brad Lewis <[email protected]> Reviewed by: Andriy Gapon <[email protected]> Approved by: Dan McDonald <[email protected]> Ported-by: Brian Behlendorf <[email protected]> OpenZFS-issue: https://illumos.org/issues/9464 OpenZFS-commit: https://github.com/openzfs/openzfs/commit/1cd7635b Closes #7587
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