A comparison of the new nfsd iomodes (and an experimental one)#925
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A comparison of the new nfsd iomodes (and an experimental one)#925vfsci-bot[bot] wants to merge 1 commit intovfs.base.cifrom
vfsci-bot[bot] wants to merge 1 commit intovfs.base.cifrom
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IOCB_DONTCACHE flushes all dirty pages on every write via
filemap_flush_range() with nr_to_write=LONG_MAX. Under concurrent
writers, this creates severe serialization: every writer contends on
the writeback submission path, leading to catastrophic throughput
collapse (~1 GB/s vs ~10 GB/s for buffered) and multi-second tail
latency.
Add IOCB_DONTCACHE_LAZY as a gentler alternative with two mechanisms:
1. Skip-if-busy: check mapping_tagged(PAGECACHE_TAG_WRITEBACK) before
flushing. If writeback is already in progress on the mapping, the
flush is skipped entirely, eliminating writeback submission
contention between concurrent writers.
2. Proportional cap: when flushing does occur, cap nr_to_write to the
number of pages just written. This prevents any single write from
triggering a full-file flush that would starve concurrent readers.
Together these mechanisms rate-limit writeback to match the incoming
write rate while avoiding I/O bursts that cause tail latency spikes.
Like IOCB_DONTCACHE, pages touched under IOCB_DONTCACHE_LAZY are
marked for eviction (dropbehind) to keep page cache usage bounded.
Also add RWF_DONTCACHE_LAZY (0x200) as a user-visible pwritev2/io_uring
flag that maps to IOCB_DONTCACHE_LAZY. The flag follows the same
validation as RWF_DONTCACHE: the filesystem must support FOP_DONTCACHE,
DAX is not supported, and RWF_DONTCACHE and RWF_DONTCACHE_LAZY are
mutually exclusive.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
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Series: https://patchwork.kernel.org/project/linux-fsdevel/list/?series=1073037
Submitter: Jeff Layton
Version: 1
Patches: 1/1
Message-ID:
<4ebbd194ccfb3bcea6225d926b4c9f339e21c813.camel@kernel.org>Base: vfs.base.ci
Lore: https://lore.kernel.org/linux-fsdevel/4ebbd194ccfb3bcea6225d926b4c9f339e21c813.camel@kernel.org
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