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Speculative prefetch for reordered requests

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Before this change speculative prefetcher was able to detect a stream
only if all of its accesses are perfectly sequential.  It was easy to
implement and is perfectly fine for single-threaded applications.
Unfortunately multi-threaded network servers, such as iSCSI, SMB or
NFS usually have plenty of threads and may often reorder requests,
preventing successful speculation and prefetch.

This change allows speculative prefetcher to detect streams even if
requests are reordered by introducing a list of 9 non-contiguous
ranges up to 16MB ahead of current stream position and filling the
gaps as more requests arrive.  It also allows stream to proceed
even with holes up to a certain configurable threshold (25%).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #16022
This commit is contained in:
Alexander Motin
2024-04-08 18:13:27 -04:00
committed by Brian Behlendorf
parent 602b5dca7b
commit 026fe79646
5 changed files with 272 additions and 63 deletions
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include/sys/dmu_zfetch.h
+11 -5
View File
@@ -45,18 +45,24 @@ typedef struct zfetch {
int zf_numstreams; /* number of zstream_t's */
} zfetch_t;
typedef struct zsrange {
uint16_t start;
uint16_t end;
} zsrange_t;
#define ZFETCH_RANGES 9 /* Fits zstream_t into 128 bytes */
typedef struct zstream {
list_node_t zs_node; /* link for zf_stream */
uint64_t zs_blkid; /* expect next access at this blkid */
uint_t zs_atime; /* time last prefetch issued */
zsrange_t zs_ranges[ZFETCH_RANGES]; /* ranges from future */
unsigned int zs_pf_dist; /* data prefetch distance in bytes */
unsigned int zs_ipf_dist; /* L1 prefetch distance in bytes */
uint64_t zs_pf_start; /* first data block to prefetch */
uint64_t zs_pf_end; /* data block to prefetch up to */
uint64_t zs_ipf_start; /* first data block to prefetch L1 */
uint64_t zs_ipf_end; /* data block to prefetch L1 up to */
list_node_t zs_node; /* link for zf_stream */
hrtime_t zs_atime; /* time last prefetch issued */
zfetch_t *zs_fetch; /* parent fetch */
boolean_t zs_missed; /* stream saw cache misses */
boolean_t zs_more; /* need more distant prefetch */
zfs_refcount_t zs_callers; /* number of pending callers */
@@ -74,7 +80,7 @@ void dmu_zfetch_init(zfetch_t *, struct dnode *);
void dmu_zfetch_fini(zfetch_t *);
zstream_t *dmu_zfetch_prepare(zfetch_t *, uint64_t, uint64_t, boolean_t,
boolean_t);
void dmu_zfetch_run(zstream_t *, boolean_t, boolean_t);
void dmu_zfetch_run(zfetch_t *, zstream_t *, boolean_t, boolean_t);
void dmu_zfetch(zfetch_t *, uint64_t, uint64_t, boolean_t, boolean_t,
boolean_t);