Hi Neil,
On Wed, Sep 10, 2014 at 12:36 PM, NeilBrown <neilb@xxxxxxx> wrote:
> On Wed, 10 Sep 2014 11:01:30 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx>
> wrote:
>
>> Hello Neil,
>>
>> On Tue, Sep 9, 2014 at 12:45 PM, NeilBrown <neilb@xxxxxxx> wrote:
>> > On Tue, 9 Sep 2014 11:33:13 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx>
>> > wrote:
>> >
>> >> Hi Neil,
>> >>
>> >>
>> >> On Tue, Sep 9, 2014 at 4:45 AM, NeilBrown <neilb@xxxxxxx> wrote:
>> >> > On Mon, 8 Sep 2014 16:55:52 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx>
>> >> > 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@xxxxxxx>
>> >> >> 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.
I am working with kernel 3.8.13. But your master branch has the same
code with respect to plug/unplug logic.
> But if it works with plugging, then just do it that way(?).
It works perfectly, and latency is much better. The only doubt is with
bitmap_unplug being called from multiple threads now. However, it can
happen for anybody that uses plug/unplug on top of MD raid1 (like ext4
for example). So question is whether it is safe for MD users to
plug/unplug when submitting bios to MD. If not, would you be fixing
this?
Alex.
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
