Re: [PATCH 6/6] md/raid5: derive the default group_thread_cnt from the hardware

["yu kuai" <yukuai@fygo.io>]

Hi,

在 2026/7/10 21:23, Hiroshi Nishida 写道:
> setup_conf() starts every array with worker_cnt_per_group = 0, i.e. a
> single raid5d thread and no worker groups.  On a many-core host backing a
> fast, wide array that single thread is the bottleneck; the multi-threading
> that group_thread_cnt enables has to be turned on by hand, per array,
> after every assembly.
>
> Pick a starting value from the CPU count instead.  When the new
> group_thread_cnt_default module parameter is left at -1 (the default),
> raid5_default_group_thread_cnt() uses half the online CPU count spread
> across the NUMA nodes, capped at 256:
>
> 	gtc = num_online_cpus() / (2 * nr_nodes)
>
> The count is per NUMA node, matching alloc_thread_groups(), so dividing by
> the node count keeps the total near half the CPUs regardless of socket
> count.  It is only a ceiling: raid5_wakeup_stripe_thread() wakes workers
> in proportion to the queued stripe count, so a lightly loaded array uses
> far fewer than the maximum.  A lone worker is not worth its overhead over
> raid5d, so a result of 1 collapses back to 0, which keeps boxes with two
> or fewer CPUs single-threaded.
>
> Measured on a 16-disk raid6 array of NVMe SSDs (a 32-vCPU host: 16 cores,
> two NUMA nodes; steady state, interleaved runs): the derived count of 8
> raises throughput 2.1-3.2x over the single-threaded default -- 4K random
> write from ~39k to ~100k IOPS (2.6x), mixed database/OLTP/high-concurrency
> 2.1-2.5x, partial stripe write 3.2x.  There is no regression on smaller
> machines: a 4-CPU box gets 2 workers and is faster or equal on every
> workload, and a box with two or fewer CPUs gets 0 and is byte-for-byte
> unchanged.
>
> group_thread_cnt_default overrides the heuristic (0 forces the historical
> single-threaded behaviour; a positive value pins a count, capped at 256),
> and the existing per-array group_thread_cnt sysfs attribute continues to
> override it at runtime and still accepts up to its own 8192 limit.
>
> Signed-off-by: Hiroshi Nishida <nishidafmly@gmail.com>
> ---
>   drivers/md/raid5.c | 45 ++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 42 insertions(+), 3 deletions(-)

There is no test and I'll not accept this patch. And again, please consider
a user space tool or a downstream patch for your production.

>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 8e1c2eba4241..6e91eb0ad575 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -88,6 +88,11 @@ module_param(stripe_cache_size_default, uint, 0644);
>   MODULE_PARM_DESC(stripe_cache_size_default,
>   		 "Initial stripe_cache_size for newly created arrays.  0 (the default) auto-sizes it from system memory: the historical 256 on small hosts, scaling up with RAM to a capped maximum on larger ones.  A non-zero value sets a fixed initial size.  Existing arrays are unaffected");
>   
> +static int group_thread_cnt_default = -1;
> +module_param(group_thread_cnt_default, int, 0644);
> +MODULE_PARM_DESC(group_thread_cnt_default,
> +		 "Initial group_thread_cnt (raid5 worker threads per NUMA node) for newly created arrays.  A negative value (the default, -1) auto-sizes it from the CPU count; 0 forces the single-threaded raid5d; a positive value sets a fixed count (capped at 256).  The per-array group_thread_cnt sysfs attribute overrides this and allows larger values");
> +
>   static bool devices_handle_discard_safely = false;
>   module_param(devices_handle_discard_safely, bool, 0644);
>   MODULE_PARM_DESC(devices_handle_discard_safely,
> @@ -7584,6 +7589,31 @@ static unsigned long raid5_cache_count(struct shrinker *shrink,
>   #define RAID5_CACHE_DEFAULT_RAM_SHIFT	9	/* above it: ~1/512 of the extra RAM */
>   #define RAID5_CACHE_DEFAULT_MAX		4096
>   
> +/*
> + * Default group_thread_cnt (worker_cnt_per_group) for a new array when the
> + * group_thread_cnt_default module parameter is left at -1.  The historical
> + * default is 0 -- a single raid5d thread -- which cannot keep a fast, wide
> + * array busy on a many-core host.  Derive a starting point from the CPU
> + * count: half the online CPUs divided across the NUMA nodes (this is a
> + * per-node count -- see alloc_thread_groups() -- so the total lands near
> + * half the online CPUs regardless of socket count), capped at
> + * RAID5_AUTO_GROUP_THREAD_MAX.  This is only a ceiling:
> + * raid5_wakeup_stripe_thread() wakes workers in proportion to the queued
> + * stripe count, so a lightly loaded array uses far fewer.  A lone worker is
> + * not worth its overhead, so 1 collapses back to 0.  The group_thread_cnt
> + * sysfs attribute overrides this per array.
> + */
> +#define RAID5_AUTO_GROUP_THREAD_MAX	256
> +
> +static int raid5_default_group_thread_cnt(void)
> +{
> +	unsigned int gtc = num_online_cpus() / (2 * num_possible_nodes());
> +
> +	if (gtc > RAID5_AUTO_GROUP_THREAD_MAX)
> +		gtc = RAID5_AUTO_GROUP_THREAD_MAX;
> +	return gtc == 1 ? 0 : gtc;
> +}
> +
>   static struct r5conf *setup_conf(struct mddev *mddev)
>   {
>   	struct r5conf *conf;
> @@ -7594,6 +7624,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
>   	int i;
>   	int group_cnt;
>   	struct r5worker_group *new_group;
> +	int def_threads;
>   	int ret = -ENOMEM;
>   
>   	if (mddev->new_level != 5
> @@ -7677,10 +7708,18 @@ static struct r5conf *setup_conf(struct mddev *mddev)
>   		goto abort;
>   	for (i = 0; i < PENDING_IO_MAX; i++)
>   		list_add(&conf->pending_data[i].sibling, &conf->free_list);
> -	/* Don't enable multi-threading by default*/
> -	if (!alloc_thread_groups(conf, 0, &group_cnt, &new_group)) {
> +	/*
> +	 * Multi-threading defaults to a hardware-derived worker count (see
> +	 * raid5_default_group_thread_cnt()); group_thread_cnt_default overrides
> +	 * the choice, and the group_thread_cnt sysfs attribute overrides it per
> +	 * array.
> +	 */
> +	def_threads = group_thread_cnt_default < 0 ?
> +		raid5_default_group_thread_cnt() :
> +		min(group_thread_cnt_default, RAID5_AUTO_GROUP_THREAD_MAX);
> +	if (!alloc_thread_groups(conf, def_threads, &group_cnt, &new_group)) {
>   		conf->group_cnt = group_cnt;
> -		conf->worker_cnt_per_group = 0;
> +		conf->worker_cnt_per_group = def_threads;
>   		conf->worker_groups = new_group;
>   	} else
>   		goto abort;

-- 
Thanks,
Kuai