[dm-devel] Raid0 performance regression

[Roger Willcocks]

> On 23 Jan 2022, at 21:34, Paul Menzel <pmenzel at molgen.mpg.de> wrote:
> 
> Dear Roger,
> 
> 
> Am 23.01.22 um 19:00 schrieb Lukas Straub:
>> CC'ing Song Liu (md-raid maintainer) and linux-raid mailing list.
>> On Fri, 21 Jan 2022 16:38:03 +0000 Roger Willcocks wrote:
> 
>>> we noticed a thirty percent drop in performance on one of our raid
>>> arrays when switching from CentOS 6.5 to 8.4; it uses raid0-like
> 
> For those outside the CentOS universe, what Linux kernel versions are those?
> 

2.6.32 (and backported changes) and 4.18.0 (sim.)

>>> striping to balance (by time) access to a pair of hardware raid-6
>>> arrays. The underlying issue is also present in the native raid0
>>> driver so herewith the gory details; I'd appreciate your thoughts.
>>> 
>>> --
>>> 
>>> blkdev_direct_IO() calls submit_bio() which calls an outermost
>>> generic_make_request() (aka submit_bio_noacct()).
>>> 
>>> md_make_request() calls blk_queue_split() which cuts an incoming
>>> request into two parts with the first no larger than get_max_io_size()
>>> bytes (which in the case of raid0, is the chunk size):
>>> 
>>>   R -> AB
>>>   blk_queue_split() gives the second part 'B' to generic_make_request()
>>> to worry about later and returns the first part 'A'.
>>> 
>>> md_make_request() then passes 'A' to a more specific request handler,
>>> In this case raid0_make_request().
>>> 
>>> raid0_make_request() cuts its incoming request into two parts at the
>>> next chunk boundary:
>>> 
>>> A -> ab
>>> 
>>> it then fixes up the device (chooses a physical device) for 'a', and
>>> gives both parts, separately, to generic make request()
>>> 
>>> This is where things go awry, because 'b' is still targetted to the
>>> original device (same as 'B'), but 'B' was queued before 'b'. So we
>>> end up with:
>>> 
>>>   R -> Bab
>>> 
>>> The outermost generic_make_request() then cuts 'B' at
>>> get_max_io_size(), and the process repeats. Ascii art follows:
>>> 
>>> 
>>>     /---------------------------------------------------/   incoming rq
>>> 
>>>     /--------/--------/--------/--------/--------/------/   max_io_size
>>>       |--------|--------|--------|--------|--------|--------|--------| chunks
>>> 
>>> |...=====|---=====|---=====|---=====|---=====|---=====|--......| rq out
>>>       a    b  c     d  e     f  g     h  i     j  k     l
>>> 
>>> Actual submission order for two-disk raid0: 'aeilhd' and 'cgkjfb'
>>> 
>>> --
>>> 
>>> There are several potential fixes -
>>> 
>>> simplest is to set raid0 blk_queue_max_hw_sectors() to UINT_MAX
>>> instead of chunk_size, so that raid0_make_request() receives the
>>> entire transfer length and cuts it up at chunk boundaries;
>>> 
>>> neatest is for raid0_make_request() to recognise that 'b' doesn't
>>> cross a chunk boundary so it can be sent directly to the physical
>>> device;
>>> 
>>> and correct is for blk_queue_split to requeue 'A' before 'B'.
>>> 
>>> --
>>> 
>>> There's also a second issue - with large raid0 chunk size (256K), the
>>> segments submitted to the physical device are at least 128K and
>>> trigger the early unplug code in blk_mq_make_request(), so the
>>> requests are never merged. There are legitimate reasons for a large
>>> chunk size so this seems unhelpful.
>>> 
>>> --
>>> 
>>> As I said, I'd appreciate your thoughts.
> 
> Thank you for the report and the analysis.
> 
> Is the second issue also a regression? If not, I suggest to split it into a separate thread.
> 

Yes this is also a regression, both issues above have to be addressed to recover the
original performance.

Specifically, an md raid0 array with 256K chunk size interleaving two x 12-disk raid6
devices (Adaptec 3154 controller, 50MB files stored contiguously on disk, four threads)
can achieve a sequential read rate of 3800 MB/sec with the (very) old 6.5 kernel; this
falls to 2500 MB/sec with the relatively newer kernel.

This change to raid0.c:

-               blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors);
+              blk_queue_max_hw_sectors(mddev->queue, UINT_MAX);

improves things somewhat, the sub-chunk requests are now submitted in order but we
still only get 2800 MB/sec because no merging takes place; the controller struggles to
keep up with the large number of sub-chunk transfers. This additional change to 
blk-mq.c:

-		if (request_count >= BLK_MAX_REQUEST_COUNT || (last &&
+		if (request_count >= BLK_MAX_REQUEST_COUNT || (false && last &&
 		    blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
 			blk_flush_plug_list(plug, false);

Brings performance back to 6.5 levels.


> 
> Kind regards,
> 
> Paul
>