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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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cmd/arc_summary
+10 -1
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@@ -793,18 +793,27 @@ def section_dmu(kstats_dict):
zfetch_stats = isolate_section('zfetchstats', kstats_dict)
zfetch_access_total = int(zfetch_stats['hits'])+int(zfetch_stats['misses'])
zfetch_access_total = int(zfetch_stats['hits']) +\
int(zfetch_stats['future']) + int(zfetch_stats['stride']) +\
int(zfetch_stats['past']) + int(zfetch_stats['misses'])
prt_1('DMU predictive prefetcher calls:', f_hits(zfetch_access_total))
prt_i2('Stream hits:',
f_perc(zfetch_stats['hits'], zfetch_access_total),
f_hits(zfetch_stats['hits']))
future = int(zfetch_stats['future']) + int(zfetch_stats['stride'])
prt_i2('Hits ahead of stream:', f_perc(future, zfetch_access_total),
f_hits(future))
prt_i2('Hits behind stream:',
f_perc(zfetch_stats['past'], zfetch_access_total),
f_hits(zfetch_stats['past']))
prt_i2('Stream misses:',
f_perc(zfetch_stats['misses'], zfetch_access_total),
f_hits(zfetch_stats['misses']))
prt_i2('Streams limit reached:',
f_perc(zfetch_stats['max_streams'], zfetch_stats['misses']),
f_hits(zfetch_stats['max_streams']))
prt_i1('Stream strides:', f_hits(zfetch_stats['stride']))
prt_i1('Prefetches issued', f_hits(zfetch_stats['io_issued']))
print()