[Performance]
1. One thread (numjobs=1) randread (bs=4k, direct=1) one dm-linear
device, which is built upon 3 nvme devices, with one polling hw
queue per nvme device.
| IOPS (IRQ mode) | IOPS (iopoll=1 mode) | diff
---- | --------------- | -------------------- | ----
dm | 208k | 279k | ~34%
2. Three threads (numjobs=3) randread (bs=4k, direct=1) one dm-linear
device, which is built upon 3 nvme devices, with one polling hw
queue per nvme device.
It's compared to 3 threads directly randread 3 nvme devices, with one
polling hw queue per nvme device. No CPU affinity set for these 3
threads. Thus every thread can access every nvme device
(filename=/dev/nvme0n1:/dev/nvme1n1:/dev/nvme2n1), i.e., every thread
need to competing for every polling hw queue.
| IOPS (IRQ mode) | IOPS (iopoll=1 mode) | diff
---- | --------------- | -------------------- | ----
dm | 615k | 728k | ~18%
nvme | 728k | 873k | ~20%
The result shows that the performance gain of bio-based polling is
comparable as that of mq polling in the same test case.
3. Three threads (numjobs=3) randread (bs=12k, direct=1) one
**dm-stripe** device, which is built upon 3 nvme devices, with one
polling hw queue per nvme device.
It's compared to 3 threads directly randread 3 nvme devices, with one
polling hw queue per nvme device. No CPU affinity set for these 3
threads. Thus every thread can access every nvme device
(filename=/dev/nvme0n1:/dev/nvme1n1:/dev/nvme2n1), i.e., every thread
need to competing for every polling hw queue.
| IOPS (IRQ mode) | IOPS (iopoll=1 mode) | diff
---- | --------------- | -------------------- | ----
dm | 314k | 354k | ~13%
nvme | 728k | 873k | ~20%
4. This patchset shall do no harm to the performance of the original mq
polling. Following is the test results of one thread (numjobs=1)
randread (bs=4k, direct=1) one nvme device.
| IOPS (IRQ mode) | IOPS (iopoll=1 mode) | diff
---------------- | --------------- | -------------------- | ----
without patchset | 242k | 332k | ~39%
with patchset | 236k | 332k | ~39%
[Changes since v2]
Patchset v2 caches all hw queues (in polling mode) of underlying mq
devices in dm layer. The polling routine actually iterates through all
these cached hw queues.
However, mq may change the queue mapping at runtime (e.g., NVMe RESET
command), thus the cached hw queues in dm layer may be out-of-date. Thus
patchset v3 falls back to the implementation of the very first RFC
version, in which the mq layer needs to export one interface iterating
all polling hw queues (patch 5), and the bio-based polling routine just
calls this interface to iterate all polling hw queues.
Besides, several new optimization is proposed.
- patch 1,2,7
same as v2, untouched
- patch 3
Considering advice from Christoph Hellwig, while refactoring blk_poll(),
split mq and bio-based polling routine from the very beginning. Now
blk_poll() is just a simple entry. blk_bio_poll() is simply copied from
blk_mq_poll(), while the loop structure is some sort of duplication
though.
- patch 4
This patch is newly added to support turning on/off polling through
'/sys/block/<dev>/queue/io_poll' dynamiclly for bio-based devices.
Patchset v2 implemented this functionality by added one new queue flag,
which is not preferred since the queue flag resource is quite short of
nowadays.
- patch 5
This patch is newly added, preparing for the following bio-based
polling. The following bio-based polling will call this helper function,
accounting on the corresponding hw queue.
- patch 6
It's from the very first RFC version, preparing for the following
bio-based polling.
- patch 8
One fixing patch needed by the following bio-based polling. It's
actually a v2 of [1]. I had sent the v2 singly in-reply-to [1], though
it has not been visible on the mailing list maybe due to the delay.
- patch 9
It's from the very first RFC version.
- patch 10
This patch is newly added. Patchset v2 had ever proposed one
optimization that, skipping the **busy** hw queues during the iteration
phase. Back upon that time, one flag of 'atomic_t' is specifically
maintained in dm layer, representing if the corresponding hw queue is
busy or not. The idea is inherited, while the implementation changes.
Now @nvmeq->cq_poll_lock is used directly here, no need for extra flag
anymore.
This optimization can significantly reduce the competition for one hw
queue between multiple polling instances. Following statistics is the
test result when 3 threads concurrently randread (bs=4k, direct=1) one
dm-linear device, which is built upon 3 nvme devices, with one polling
hw queue per nvme device.
| IOPS (IRQ mode) | IOPS (iopoll=1 mode) | diff
----------- | --------------- | -------------------- | ----
without opt | 318k | 256k | ~-20%
with opt | 314k | 354k | ~13%
- patch 11
This is another newly added optimizatin for bio-based polling.
One intuitive insight is that, when the original bio submitted to dm
device doesn't get split, then the bio gets enqueued into only one hw
queue of one of the underlying mq devices. In this case, we no longer
need to track all split bios, and one cookie (for the only split bio)
is enough. It is implemented by returning the pointer to the
corresponding hw queue in this case.
It should be safe by directly returning the pointer to the hw queue,
since 'struct blk_mq_hw_ctx' won't be freed during the whole lifetime of
'struct request_queue'. Even when the number of hw queues may decrease
when NVMe RESET happens, the 'struct request_queue' structure of decreased
hw queues won't be freed, instead it's buffered into
&q->unused_hctx_list list.
Though this optimization seems quite intuitive, the performance test
shows that it does no benefit nor harm to the performance, while 3
threads concurrently randreading (bs=4k, direct=1) one dm-linear
device, which is built upon 3 nvme devices, with one polling hw queue
per nvme device.
I'm not sure why it doesn't work, maybe because the number of devices,
or the depth of the devcice stack is to low in my test case?
[Remained Issue]
It has been mentioned in patch 4 that, users could change the state of
the underlying devices through '/sys/block/<dev>/io_poll', bypassing
the dm device above. Thus it can cause a situation where QUEUE_FLAG_POLL
is still set for the request_queue of dm device, while one of the
underlying mq device may has cleared this flag.
In this case, it will pass the 'test_bit(QUEUE_FLAG_POLL, &q->queue_flags)'
check in blk_poll(), while the input cookie may actually points to a hw
queue in IRQ mode since patch 11. Thus for this hw queue (in IRQ mode),
the bio-based polling routine will handle this hw queue acquiring
'spin_lock(&nvmeq->cq_poll_lock)' (refer
drivers/nvme/host/pci.c:nvme_poll), which is not adequate since this hw
queue may also be accessed in IRQ context. In other words,
spin_lock_irq() should be used here.
I have not come up one simple way to fix it. I don't want to do sanity
check (e.g., the type of the hw queue is HCTX_TYPE_POLL or not) in the
IO path (submit_bio()/blk_poll()), i.e., fast path.
We'd better fix it in the control path, i.e., dm could be aware of the
change when attribute (e.g., support io_poll or not) of one of the
underlying devices changed at runtime.
[1] https://listman.redhat.com/archives/dm-devel/2021-February/msg00028.html
changes since v1:
- patch 1,2,4 is the same as v1 and have already been reviewed
- patch 3 is refactored a bit on the basis of suggestions from
Mike Snitzer.
- patch 5 is newly added and introduces one new queue flag
representing if the queue is capable of IO polling. This mainly
simplifies the logic in queue_poll_store().
- patch 6 implements the core mechanism supporting IO polling.
The sanity check checking if the dm device supports IO polling is
also folded into this patch, and the queue flag will be cleared if
it doesn't support, in case of table reloading.
Jeffle Xu (11):
block: move definition of blk_qc_t to types.h
block: add queue_to_disk() to get gendisk from request_queue
block: add poll method to support bio-based IO polling
block: add poll_capable method to support bio-based IO polling
block/mq: extract one helper function polling hw queue
block/mq: add iterator for polling hw queues
dm: always return BLK_QC_T_NONE for bio-based device
dm: fix iterate_device sanity check
dm: support IO polling for bio-based dm device
nvme/pci: don't wait for locked polling queue
dm: fastpath of bio-based polling
block/blk-core.c | 89 +++++++++++++-
block/blk-mq.c | 27 +----
block/blk-sysfs.c | 14 ++-
drivers/md/dm-table.c | 215 ++++++++++++++++++----------------
drivers/md/dm.c | 90 +++++++++++---
drivers/md/dm.h | 2 +-
drivers/nvme/host/pci.c | 4 +-
include/linux/blk-mq.h | 21 ++++
include/linux/blk_types.h | 10 +-
include/linux/blkdev.h | 4 +
include/linux/device-mapper.h | 1 +
include/linux/fs.h | 2 +-
include/linux/types.h | 3 +
include/trace/events/kyber.h | 6 +-
14 files changed, 333 insertions(+), 155 deletions(-)
--
2.27.0
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