| Message ID | 20191009032124.10541-2-david@fromorbit.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | mm, xfs: non-blocking inode reclaim | expand |
On Wed, Oct 09, 2019 at 02:20:59PM +1100, 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> > --- Same as the previous, yes..? 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 >
On Wed, Oct 09, 2019 at 02:20:59PM +1100, 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> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> --D > --- > 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 >
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