Re: Question about RAID1 plug/unplug code

[NeilBrown <neilb@suse.de>]

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On Wed, 10 Sep 2014 11:01:30 +0300 Alexander Lyakas <alex.bolshoy@gmail.com>
wrote:

> Hello Neil,
> 
> On Tue, Sep 9, 2014 at 12:45 PM, NeilBrown <neilb@suse.de> wrote:
> > On Tue, 9 Sep 2014 11:33:13 +0300 Alexander Lyakas <alex.bolshoy@gmail.com>
> > wrote:
> >
> >> Hi Neil,
> >>
> >>
> >> On Tue, Sep 9, 2014 at 4:45 AM, NeilBrown <neilb@suse.de> wrote:
> >> > On Mon, 8 Sep 2014 16:55:52 +0300 Alexander Lyakas <alex.bolshoy@gmail.com>
> >> > wrote:
> >> >
> >> >> Hi Neil,
> >> >> We have been seeing high latency on the md/raid1 block device, due to
> >> >> the fact that all WRITEs are handed off to raid1d thread. This thread
> >> >> also calls bitmap_unplug(), which writes the bitmap synchronously.
> >> >> While it waits for the bitmap, it cannot trigger other WRITEs waiting
> >> >> in its pending_bio_list. This is especially seen with SSDs: MD's
> >> >> latency is much higher that SSD latency (I have been stoned by Peter
> >> >> Grandi when I brought up this issue previously for raid5).
> >> >>
> >> >> Then I have noticed the commit:
> >> >>
> >> >> commit f54a9d0e59c4bea3db733921ca9147612a6f292c
> >> >> Author: NeilBrown <neilb@suse.de>
> >> >> Date:   Thu Aug 2 08:33:20 2012 +1000
> >> >>
> >> >>     md/raid1: submit IO from originating thread instead of md thread.
> >> >>
> >> >> Looking at the code, I learned that to avoid switching into raid1d,
> >> >> the caller has to use blk_start_plug/blk_finish_plug. So I added these
> >> >> calls in our kernel module, which submits bios to MD. Results were
> >> >> awesome, MD latency got down significantly.
> >> >
> >> > That's good to hear.
> >> >
> >> >>
> >> >> So I have several questions about this plug/unplug thing.
> >> >>
> >> >> 1/ Originally this infrastructure was supposed to help IO schedulers
> >> >> in merging requests. It is useful when one has a bunch of requests to
> >> >> submit in one shot.
> >> >
> >> > That is exactly the whole point of plugging:  allow the device to handle a
> >> > batch of requests together instead of one at a time.
> >> >
> >> >> But in MD case, thus infrastructure is used for a different purpose:
> >> >> not to merge requests (which may help bandwidth, but probably not
> >> >> latency), but to avoid making raid1d a bottleneck, to be able to
> >> >> submit requests from multiple threads in parallel, which brings down
> >> >> latency significantly in our case. Indeed "struct blk_plug" has a
> >> >> special "cb_list", which is used only by MD.
> >> >
> >> > I don't think the way md uses plugging is conceptually different from any
> >> > other use: it is always about gathering a batch together.
> >> > "cb_list" is handled by blk_check_plugged() which is also used by
> >> > block/umem.c and btrfs.
> >> >
> >> > The base plugging code assumes that it is only gathering a batch of requests
> >> > for a single device - if the target device changes then the batch is flushed.
> >> > It also assumed that it was "struct request" that was batched.
> >> > Devices like md that want to queue 'struct bio', something else was needed.
> >> > Also with layered devices it can be useful to gather multiple batches for
> >> > multiple layers.
> >> > So I created "cb_list" etc and a more generic interface.
> >> >
> >> >> In my case I have only individual bios (not a bunch of bios), and I
> >> >> after wrap them with plug/unplug, MD latency gets better. So we are
> >> >> using the plug infrastructure for a different purpose.
> >> >> Is my understanding correct? Was this your intention?
> >> >
> >> > I don't really understand what you are doing.  There is no point in using
> >> > plugging for individual bios.  The  main point for raid1 writes is to gather
> >> > a lot of writes together so that all multiple bitmap bits can be set all at
> >> > once.
> >> > It should be possible to submit individual bios directly from make_request
> >> > without passing them to raid1d and without using plugging.
> >> Can you pls explain how it is possible?
> >> You have this code for WRITEs:
> >>         cb = blk_check_plugged(raid1_unplug, mddev, sizeof(*plug));
> >>         if (cb)
> >>             plug = container_of(cb, struct raid1_plug_cb, cb);
> >>         else
> >>             plug = NULL;
> >>         spin_lock_irqsave(&conf->device_lock, flags);
> >>         if (plug) {
> >>             bio_list_add(&plug->pending, mbio);
> >>             plug->pending_cnt++;
> >>         } else {
> >>             bio_list_add(&conf->pending_bio_list, mbio);
> >>             conf->pending_count++;
> >>         }
> >>         spin_unlock_irqrestore(&conf->device_lock, flags);
> >>
> >> If the thread blk_check_plugged returns NULL, then you always hand the
> >> WRITE to raid1d. So the only option to avoid handoff to raid1d is for
> >> the caller to plug. Otherwise, all WRITEs are handed off to raid1d and
> >> latency becomes terrible.
> >> So in my case, I use plug/unplug for individual bios only to avoid the
> >> handoff to raid1d.
> >> What am I missing in this analysis?
> >
> > if blk_check_plugged succeeds then it has arranged for raid1_unplug to be
> > called a little later by that same process.
> > So there is nothing to stop you calling raid1_unplug immediately.
> >
> > raid1_unplug essentially does:
> >   bitmap_unplug()
> >   generic_make_request()
> >
> > so you can very nearly just do that, without any plugging.
> I am sorry, but I did not understand your reply. Maybe I did not
> explain myself, I will try again.
> 
> I am not changing raid1.c code. I just want to avoid the handoff to
> raid1d on WRITEs. According to your code, there are only two possible
> flows:
> 
> Flow 1 - with plugging
> # caller calls blk_start_plug
> # caller calls submit_bio
> # blk_check_plugged succeeds, and bio is put onto plug->pending list
> # caller calls blk_finish_plug
> # raid1_unplug is called in the same caller's thread, so it does
> bitmap_unplug and generic_make_request
> 
> Flow 2 - without plugging
> # caller calls submit_bio
> # blk_check_plugged fails, and bio is put onto conf->pending_bio_list,
> which means it will be submitted by raid1d
> 
> My conclusion from that: to avoid the handoff to raid1, caller always
> need to plug, even if it has a single bio to submit. But you said "it
> should be possible to submit individual bios directly from
> make_request without passing them to raid1d and without using
> plugging". So can you explain how it is possible? I prefer not to
> change raid1.c code.
> 
> >
> > There is a bit of extra subtlety but I can't really know how relevant that
> > might be to you without actually seeing you code.
> My code (in a different kernel module, not in raid1.c) is simply doing
> submit_bio. I want to wrap this with plug/unplug to avoid the handoff
> to raid1d and improve raid1 latency.
> 

I think I need to see the code you are working with to be able to suggest
anything used.
But if it works with plugging, then just do it that way(?).

NeilBrown

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