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There is a lock contention on device_lock in read_one_chunk().
device_lock is taken to sync conf->active_aligned_reads and
conf->quiesce.
read_one_chunk() takes the lock, then waits for quiesce=0 (resumed)
before incrementing active_aligned_reads.
raid5_quiesce() takes the lock, sets quiesce=2 (in-progress), then waits
for active_aligned_reads to be zero before setting quiesce=1
(suspended).
Introduce a fast (lockless) path in read_one_chunk(): activate aligned
read without taking device_lock. In case quiesce starts while
activating the aligned-read in fast path, deactivate it and revert to
old behavior (take device_lock and wait for quiesce to finish).
Add smp store/load in raid5_quiesce()/read_one_chunk() respectively to
gaurantee that read_one_chunk() does not miss an ongoing quiesce.
My setups:
1. 8 local nvme drives (each up to 250k iops).
2. 8 ram disks (brd).
Each setup with raid6 (6+2), 1024 io threads on a 96 cpu-cores (48 per
socket) system. Record both iops and cpu spent on this contention with
rand-read-4k. Record bw with sequential-read-128k. Note: in most cases
cpu is still busy but due to "new" bottlenecks.
nvme:
| iops | cpu | bw
-----------------------------------------------
without patch | 1.6M | ~50% | 5.5GB/s
with patch | 2M (throttled) | 0% | 16GB/s (throttled)
ram (brd):
| iops | cpu | bw
-----------------------------------------------
without patch | 2M | ~80% | 24GB/s
with patch | 4M | 0% | 55GB/s
CC: Song Liu <song@kernel.org>
CC: Neil Brown <neilb@suse.de>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Gal Ofri <gal.ofri@storing.io>
Signed-off-by: Song Liu <song@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.