Re: [patch 2/2 v3]raid5: create multiple threads to handle stripes

[Stan Hoeppner <stan@hardwarefreak.com>]

On 3/11/2013 8:39 PM, NeilBrown wrote:
> On Thu, 7 Mar 2013 15:31:23 +0800 Shaohua Li <shli@kernel.org> wrote:
...
>>> #echo 1-3 > /sys/block/md0/md/auxth0/cpulist
>>> This will bind auxiliary thread 0 to cpu 1-3, and this thread will only handle
>>> stripes produced by cpu 1-3. User tool can further change the thread's
>>> affinity, but the thread can only handle stripes produced by cpu 1-3 till the
>>> sysfs entry is changed again.

Would it not be better to use the existing cpusets infrastructure for
this, instead of manually binding threads to specific cores or sets of
cores?

Also, I understand the hot cache issue driving the desire to have a raid
thread only process stripes created by its CPU.  But what of the
scenario where an HPC user pins application threads to cores and needs
all the L1/L2 cache?  Say this user has a dual socket 24 core NUMA
system with 2 NUMA nodes per socket, 4 nodes total.  Each NUMA node has
6 cores and shared L3 cache.  The user pins 5 processes to 5 cores in
each node, and wants to pin a raid thread to the remaining core in each
node to handle the write IO generated by the 5 user threads on the node.

Does your patch series allow this?  Using the above example, if the user
creates 4 cpusets, can he assign a raid thread to that set and the
thread will execute on any core in the set, and only that set, on any
stripes created by any CPU in that set, and only that set?

The infrastructure for this already exists, has since 2004.  And it
seems is more flexible than what you've implemented here.  I suggest we
make use of it, as it is the kernel standard for doing such things.

See:  http://man7.org/linux/man-pages/man7/cpuset.7.html

> Hi Shaohua,
>  I still have this sitting in my queue, but I haven't had a chance to look at
>  is properly yet - I'm sorry about that.  I'll try to get to it soon.

-- 
Stan