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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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man/man4/zfs.4
+11
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@@ -544,6 +544,10 @@ However, this is limited by
Maximum micro ZAP size.
A micro ZAP is upgraded to a fat ZAP, once it grows beyond the specified size.
.
.It Sy zfetch_hole_shift Ns = Ns Sy 2 Pq uint
Log2 fraction of holes in speculative prefetch stream allowed for it to
proceed.
.
.It Sy zfetch_min_distance Ns = Ns Sy 4194304 Ns B Po 4 MiB Pc Pq uint
Min bytes to prefetch per stream.
Prefetch distance starts from the demand access size and quickly grows to
@@ -558,6 +562,13 @@ Max bytes to prefetch per stream.
.It Sy zfetch_max_idistance Ns = Ns Sy 67108864 Ns B Po 64 MiB Pc Pq uint
Max bytes to prefetch indirects for per stream.
.
.It Sy zfetch_max_reorder Ns = Ns Sy 16777216 Ns B Po 16 MiB Pc Pq uint
Requests within this byte distance from the current prefetch stream position
are considered parts of the stream, reordered due to parallel processing.
Such requests do not advance the stream position immediately unless
.Sy zfetch_hole_shift
fill threshold is reached, but saved to fill holes in the stream later.
.
.It Sy zfetch_max_streams Ns = Ns Sy 8 Pq uint
Max number of streams per zfetch (prefetch streams per file).
.