Re: [PATCH 1/2] xfs: Lower CIL flush limit for large logs

[Brian Foster <bfoster@redhat.com>]

On Mon, Sep 30, 2019 at 04:03:43PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> The current CIL size aggregation limit is 1/8th the log size. This
> means for large logs we might be aggregating at least 250MB of dirty objects
> in memory before the CIL is flushed to the journal. With CIL shadow
> buffers sitting around, this means the CIL is often consuming >500MB
> of temporary memory that is all allocated under GFP_NOFS conditions.
> 
> Flushing the CIL can take some time to do if there is other IO
> ongoing, and can introduce substantial log force latency by itself.
> It also pins the memory until the objects are in the AIL and can be
> written back and reclaimed by shrinkers. Hence this threshold also
> tends to determine the minimum amount of memory XFS can operate in
> under heavy modification without triggering the OOM killer.
> 
> Modify the CIL space limit to prevent such huge amounts of pinned
> metadata from aggregating. We can have 2MB of log IO in flight at
> once, so limit aggregation to 16x this size. This threshold was
> chosen as it little impact on performance (on 16-way fsmark) or log
> traffic but pins a lot less memory on large logs especially under
> heavy memory pressure.  An aggregation limit of 8x had 5-10%
> performance degradation and a 50% increase in log throughput for
> the same workload, so clearly that was too small for highly
> concurrent workloads on large logs.
> 
> This was found via trace analysis of AIL behaviour. e.g. insertion
> from a single CIL flush:
> 
> xfs_ail_insert: old lsn 0/0 new lsn 1/3033090 type XFS_LI_INODE flags IN_AIL
> 
> $ grep xfs_ail_insert /mnt/scratch/s.t |grep "new lsn 1/3033090" |wc -l
> 1721823
> $
> 
> So there were 1.7 million objects inserted into the AIL from this
> CIL checkpoint, the first at 2323.392108, the last at 2325.667566 which
> was the end of the trace (i.e. it hadn't finished). Clearly a major
> problem.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Seems reasonable:

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/xfs_log_priv.h | 29 +++++++++++++++++++++++------
>  1 file changed, 23 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index b880c23cb6e4..a3cc8a9a16d9 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -323,13 +323,30 @@ struct xfs_cil {
>   * tries to keep 25% of the log free, so we need to keep below that limit or we
>   * risk running out of free log space to start any new transactions.
>   *
> - * In order to keep background CIL push efficient, we will set a lower
> - * threshold at which background pushing is attempted without blocking current
> - * transaction commits.  A separate, higher bound defines when CIL pushes are
> - * enforced to ensure we stay within our maximum checkpoint size bounds.
> - * threshold, yet give us plenty of space for aggregation on large logs.
> + * In order to keep background CIL push efficient, we only need to ensure the
> + * CIL is large enough to maintain sufficient in-memory relogging to avoid
> + * repeated physical writes of frequently modified metadata. If we allow the CIL
> + * to grow to a substantial fraction of the log, then we may be pinning hundreds
> + * of megabytes of metadata in memory until the CIL flushes. This can cause
> + * issues when we are running low on memory - pinned memory cannot be reclaimed,
> + * and the CIL consumes a lot of memory. Hence we need to set an upper physical
> + * size limit for the CIL that limits the maximum amount of memory pinned by the
> + * CIL but does not limit performance by reducing relogging efficiency
> + * significantly.
> + *
> + * As such, the CIL push threshold ends up being the smaller of two thresholds:
> + * - a threshold large enough that it allows CIL to be pushed and progress to be
> + *   made without excessive blocking of incoming transaction commits. This is
> + *   defined to be 12.5% of the log space - half the 25% push threshold of the
> + *   AIL.
> + * - small enough that it doesn't pin excessive amounts of memory but maintains
> + *   close to peak relogging efficiency. This is defined to be 16x the iclog
> + *   buffer window (32MB) as measurements have shown this to be roughly the
> + *   point of diminishing performance increases under highly concurrent
> + *   modification workloads.
>   */
> -#define XLOG_CIL_SPACE_LIMIT(log)	(log->l_logsize >> 3)
> +#define XLOG_CIL_SPACE_LIMIT(log)	\
> +	min_t(int, (log)->l_logsize >> 3, BBTOB(XLOG_TOTAL_REC_SHIFT(log)) << 4)
>  
>  /*
>   * ticket grant locks, queues and accounting have their own cachlines
> -- 
> 2.23.0.rc1
>