Re: [PATCH 08/45] xfs: journal IO cache flush reductions

Brian Foster
2021-03-08T12:26:11Z

On Fri, Mar 05, 2021 at 04:11:06PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Currently every journal IO is issued as REQ_PREFLUSH | REQ_FUA to
> guarantee the ordering requirements the journal has w.r.t. metadata
> writeback. THe two ordering constraints are:
>
> 1. we cannot overwrite metadata in the journal until we guarantee
> that the dirty metadata has been written back in place and is
> stable.
>
> 2. we cannot write back dirty metadata until it has been written to
> the journal and guaranteed to be stable (and hence recoverable) in
> the journal.
>
> The ordering guarantees of #1 are provided by REQ_PREFLUSH. This
> causes the journal IO to issue a cache flush and wait for it to
> complete before issuing the write IO to the journal. Hence all
> completed metadata IO is guaranteed to be stable before the journal
> overwrites the old metadata.
>
> The ordering guarantees of #2 are provided by the REQ_FUA, which
> ensures the journal writes do not complete until they are on stable
> storage. Hence by the time the last journal IO in a checkpoint
> completes, we know that the entire checkpoint is on stable storage
> and we can unpin the dirty metadata and allow it to be written back.
>
> This is the mechanism by which ordering was first implemented in XFS
> way back in 2002 by commit 95d97c36e5155075ba2eb22b17562cfcc53fcf96
> ("Add support for drive write cache flushing") in the xfs-archive
> tree.
>
> A lot has changed since then, most notably we now use delayed
> logging to checkpoint the filesystem to the journal rather than
> write each individual transaction to the journal. Cache flushes on
> journal IO are necessary when individual transactions are wholly
> contained within a single iclog. However, CIL checkpoints are single
> transactions that typically span hundreds to thousands of individual
> journal writes, and so the requirements for device cache flushing
> have changed.
>
> That is, the ordering rules I state above apply to ordering of
> atomic transactions recorded in the journal, not to the journal IO
> itself. Hence we need to ensure metadata is stable before we start
> writing a new transaction to the journal (guarantee #1), and we need
> to ensure the entire transaction is stable in the journal before we
> start metadata writeback (guarantee #2).
>
> Hence we only need a REQ_PREFLUSH on the journal IO that starts a
> new journal transaction to provide #1, and it is not on any other
> journal IO done within the context of that journal transaction.
>
> The CIL checkpoint already issues a cache flush before it starts
> writing to the log, so we no longer need the iclog IO to issue a
> REQ_REFLUSH for us. Hence if XLOG_START_TRANS is passed
> to xlog_write(), we no longer need to mark the first iclog in
> the log write with REQ_PREFLUSH for this case. As an added bonus,
> this ordering mechanism works for both internal and external logs,
> meaning we can remove the explicit data device cache flushes from
> the iclog write code when using external logs.
>
> Given the new ordering semantics of commit records for the CIL, we
> need iclogs containing commit records to issue a REQ_PREFLUSH. We
> also require unmount records to do this. Hence for both
> XLOG_COMMIT_TRANS and XLOG_UNMOUNT_TRANS xlog_write() calls we need
> to mark the first iclog being written with REQ_PREFLUSH.
>
> For both commit records and unmount records, we also want them
> immediately on stable storage, so we want to also mark the iclogs
> that contain these records to be marked REQ_FUA. That means if a
> record is split across multiple iclogs, they are all marked REQ_FUA
> and not just the last one so that when the transaction is completed
> all the parts of the record are on stable storage.
>
> And for external logs, unmount records need a pre-write data device
> cache flush similar to the CIL checkpoint cache pre-flush as the
> internal iclog write code does not do this implicitly anymore.
>
> As an optimisation, when the commit record lands in the same iclog
> as the journal transaction starts, we don't need to wait for
> anything and can simply use REQ_FUA to provide guarantee #2. This
> means that for fsync() heavy workloads, the cache flush behaviour is
> completely unchanged and there is no degradation in performance as a
> result of optimise the multi-IO transaction case.
>
> The most notable sign that there is less IO latency on my test
> machine (nvme SSDs) is that the "noiclogs" rate has dropped
> substantially. This metric indicates that the CIL push is blocking
> in xlog_get_iclog_space() waiting for iclog IO completion to occur.
> With 8 iclogs of 256kB, the rate is appoximately 1 noiclog event to
> every 4 iclog writes. IOWs, every 4th call to xlog_get_iclog_space()
> is blocking waiting for log IO. With the changes in this patch, this
> drops to 1 noiclog event for every 100 iclog writes. Hence it is
> clear that log IO is completing much faster than it was previously,
> but it is also clear that for large iclog sizes, this isn't the
> performance limiting factor on this hardware.
>
> With smaller iclogs (32kB), however, there is a sustantial
> difference. With the cache flush modifications, the journal is now
> running at over 4000 write IOPS, and the journal throughput is
> largely identical to the 256kB iclogs and the noiclog event rate
> stays low at about 1:50 iclog writes. The existing code tops out at
> about 2500 IOPS as the number of cache flushes dominate performance
> and latency. The noiclog event rate is about 1:4, and the
> performance variance is quite large as the journal throughput can
> fall to less than half the peak sustained rate when the cache flush
> rate prevents metadata writeback from keeping up and the log runs
> out of space and throttles reservations.
>
> As a result:
>
> logbsize fsmark create rate rm -rf
> before 32kb 152851+/-5.3e+04 5m28s
> patched 32kb 221533+/-1.1e+04 5m24s
>
> before 256kb 220239+/-6.2e+03 4m58s
> patched 256kb 228286+/-9.2e+03 5m06s
>
> The rm -rf times are included because I ran them, but the
> differences are largely noise. This workload is largely metadata
> read IO latency bound and the changes to the journal cache flushing
> doesn't really make any noticable difference to behaviour apart from
> a reduction in noiclog events from background CIL pushing.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Thoughts on my previous feedback to this patch, particularly the locking
bits..? I thought I saw a subsequent patch somewhere that increased the
parallelism of this code..

Brian

> fs/xfs/xfs_log.c | 53 +++++++++++++++++++++++--------------------
> fs/xfs/xfs_log_cil.c | 7 +++++-
> fs/xfs/xfs_log_priv.h | 4 ++++
> 3 files changed, 38 insertions(+), 26 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 364694a83de6..ed44d67d7099 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -835,6 +835,14 @@ xlog_write_unmount_record(
>
> /* account for space used by record data */
> ticket->t_curr_res -= sizeof(ulf);
> +
> + /*
> + * For external log devices, we need to flush the data device cache
> + * first to ensure all metadata writeback is on stable storage before we
> + * stamp the tail LSN into the unmount record.
> + */
> + if (log->l_targ != log->l_mp->m_ddev_targp)
> + blkdev_issue_flush(log->l_targ->bt_bdev);
> return xlog_write(log, &vec, ticket, NULL, NULL, XLOG_UNMOUNT_TRANS);
> }
>
> @@ -1753,8 +1761,7 @@ xlog_write_iclog(
> struct xlog *log,
> struct xlog_in_core *iclog,
> uint64_t bno,
> - unsigned int count,
> - bool need_flush)
> + unsigned int count)
> {
> ASSERT(bno < log->l_logBBsize);
>
> @@ -1792,10 +1799,12 @@ xlog_write_iclog(
> * writeback throttle from throttling log writes behind background
> * metadata writeback and causing priority inversions.
> */
> - iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC |
> - REQ_IDLE | REQ_FUA;
> - if (need_flush)
> + iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC | REQ_IDLE;
> + if (iclog->ic_flags & XLOG_ICL_NEED_FLUSH)
> iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
> + if (iclog->ic_flags & XLOG_ICL_NEED_FUA)
> + iclog->ic_bio.bi_opf |= REQ_FUA;
> + iclog->ic_flags &= ~(XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA);
>
> if (xlog_map_iclog_data(&iclog->ic_bio, iclog->ic_data, count)) {
> xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR);
> @@ -1898,7 +1907,6 @@ xlog_sync(
> unsigned int roundoff; /* roundoff to BB or stripe */
> uint64_t bno;
> unsigned int size;
> - bool need_flush = true, split = false;
>
> ASSERT(atomic_read(&iclog->ic_refcnt) == 0);
>
> @@ -1923,10 +1931,8 @@ xlog_sync(
> bno = BLOCK_LSN(be64_to_cpu(iclog->ic_header.h_lsn));
>
> /* Do we need to split this write into 2 parts? */
> - if (bno + BTOBB(count) > log->l_logBBsize) {
> + if (bno + BTOBB(count) > log->l_logBBsize)
> xlog_split_iclog(log, &iclog->ic_header, bno, count);
> - split = true;
> - }
>
> /* calculcate the checksum */
> iclog->ic_header.h_crc = xlog_cksum(log, &iclog->ic_header,
> @@ -1947,22 +1953,8 @@ xlog_sync(
> be64_to_cpu(iclog->ic_header.h_lsn));
> }
> #endif
> -
> - /*
> - * Flush the data device before flushing the log to make sure all meta
> - * data written back from the AIL actually made it to disk before
> - * stamping the new log tail LSN into the log buffer. For an external
> - * log we need to issue the flush explicitly, and unfortunately
> - * synchronously here; for an internal log we can simply use the block
> - * layer state machine for preflushes.
> - */
> - if (log->l_targ != log->l_mp->m_ddev_targp || split) {
> - blkdev_issue_flush(log->l_mp->m_ddev_targp->bt_bdev);
> - need_flush = false;
> - }
> -
> xlog_verify_iclog(log, iclog, count);
> - xlog_write_iclog(log, iclog, bno, count, need_flush);
> + xlog_write_iclog(log, iclog, bno, count);
> }
>
> /*
> @@ -2416,10 +2408,21 @@ xlog_write(
> ASSERT(log_offset <= iclog->ic_size - 1);
> ptr = iclog->ic_datap + log_offset;
>
> - /* start_lsn is the first lsn written to. That's all we need. */
> + /* Start_lsn is the first lsn written to. */
> if (start_lsn && !*start_lsn)
> *start_lsn = be64_to_cpu(iclog->ic_header.h_lsn);
>
> + /*
> + * iclogs containing commit records or unmount records need
> + * to issue ordering cache flushes and commit immediately
> + * to stable storage to guarantee journal vs metadata ordering
> + * is correctly maintained in the storage media.
> + */
> + if (optype & (XLOG_COMMIT_TRANS | XLOG_UNMOUNT_TRANS)) {
> + iclog->ic_flags |= (XLOG_ICL_NEED_FLUSH |
> + XLOG_ICL_NEED_FUA);
> + }
> +
> /*
> * This loop writes out as many regions as can fit in the amount
> * of space which was allocated by xlog_state_get_iclog_space().
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index c04d5d37a3a2..263c8d907221 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -896,11 +896,16 @@ xlog_cil_push_work(
>
> /*
> * If the checkpoint spans multiple iclogs, wait for all previous
> - * iclogs to complete before we submit the commit_iclog.
> + * iclogs to complete before we submit the commit_iclog. If it is in the
> + * same iclog as the start of the checkpoint, then we can skip the iclog
> + * cache flush because there are no other iclogs we need to order
> + * against.
> */
> if (ctx->start_lsn != commit_lsn) {
> spin_lock(&log->l_icloglock);
> xlog_wait_on_iclog(commit_iclog->ic_prev);
> + } else {
> + commit_iclog->ic_flags &= ~XLOG_ICL_NEED_FLUSH;
> }
>
> /* release the hounds! */
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 56e1942c47df..0552e96d2b64 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -133,6 +133,9 @@ enum xlog_iclog_state {
>
> #define XLOG_COVER_OPS 5
>
> +#define XLOG_ICL_NEED_FLUSH (1 << 0) /* iclog needs REQ_PREFLUSH */
> +#define XLOG_ICL_NEED_FUA (1 << 1) /* iclog needs REQ_FUA */
> +
> /* Ticket reservation region accounting */
> #define XLOG_TIC_LEN_MAX 15
>
> @@ -201,6 +204,7 @@ typedef struct xlog_in_core {
> u32 ic_size;
> u32 ic_offset;
> enum xlog_iclog_state ic_state;
> + unsigned int ic_flags;
> char *ic_datap; /* pointer to iclog data */
>
> /* Callback structures need their own cacheline */
> --
> 2.28.0
>