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SubjectRe: [PATCH 3/3] writeback: add dirty_ratio_time per bdi variable (NFS write performance)
From
2012/8/20, Fengguang Wu <fengguang.wu@intel.com>:
> On Mon, Aug 20, 2012 at 09:48:42AM +0900, Namjae Jeon wrote:
>> 2012/8/19, Fengguang Wu <fengguang.wu@intel.com>:
>> > On Sat, Aug 18, 2012 at 05:50:02AM -0400, Namjae Jeon wrote:
>> >> From: Namjae Jeon <namjae.jeon@samsung.com>
>> >>
>> >> This patch is based on suggestion by Wu Fengguang:
>> >> https://lkml.org/lkml/2011/8/19/19
>> >>
>> >> kernel has mechanism to do writeback as per dirty_ratio and
>> >> dirty_background
>> >> ratio. It also maintains per task dirty rate limit to keep balance of
>> >> dirty pages at any given instance by doing bdi bandwidth estimation.
>> >>
>> >> Kernel also has max_ratio/min_ratio tunables to specify percentage of
>> >> writecache
>> >> to control per bdi dirty limits and task throtelling.
>> >>
>> >> However, there might be a usecase where user wants a writeback tuning
>> >> parameter to flush dirty data at desired/tuned time interval.
>> >>
>> >> dirty_background_time provides an interface where user can tune
>> >> background
>> >> writeback start time using /sys/block/sda/bdi/dirty_background_time
>> >>
>> >> dirty_background_time is used alongwith average bdi write bandwidth
>> >> estimation
>> >> to start background writeback.
>> >
>> > Here lies my major concern about dirty_background_time: the write
>> > bandwidth estimation is an _estimation_ and will sure become wildly
>> > wrong in some cases. So the dirty_background_time implementation based
>> > on it will not always work to the user expectations.
>> >
>> > One important case is, some users (eg. Dave Chinner) explicitly take
>> > advantage of the existing behavior to quickly create & delete a big
>> > 1GB temp file without worrying about triggering unnecessary IOs.
>> >
>> Hi. Wu.
>> Okay, I have a question.
>>
>> If making dirty_writeback_interval per bdi to tune short interval
>> instead of background_time, We can get similar performance
>> improvement.
>> /sys/block/<device>/bdi/dirty_writeback_interval
>> /sys/block/<device>/bdi/dirty_expire_interval
>>
>> NFS write performance improvement is just one usecase.
>>
>> If we can set interval/time per bdi, other usecases will be created
>> by applying.
>
> Per-bdi interval/time tunables, if there comes such a need, will in
> essential be for data caching and safety. If turning them into some
> requirement for better performance, the users will potential be
> stretched on choosing the "right" value for balanced data cache,
> safety and performance. Hmm, not a comfortable prospection.
Hi Wu.
First, Thanks for shared information.

I change writeback interval on NFS server only.

I think that this does not affect data cache/page behaviour(caching)
change on NFS client. NFS client will start sending write requests as
per default NFS/writeback logic. So, no change in NFS client data
caching behaviour.

Also, on NFS server it does not make change in system-wide caching
behaviour. It only modifies caching/writeback behaviour of a
particular “bdi” on NFS server so that NFS client could see better
WRITE speed.

I will share several performancetest results as Dave's opinion.

>
>> >The numbers are impressive! FYI, I tried another NFS specific approach
>> >to avoid big NFS COMMITs, which achieved similar performance gains:
>>
>> >nfs: writeback pages wait queue
>> >https://lkml.org/lkml/2011/10/20/235
This patch looks client side optimization to me.(need to check more)
Do we need the optimization of server side as Bruce's opinion ?

Thanks.
>>
>> Thanks.
>
> The NFS write queue, on the other hand, is directly aimed for
> improving NFS performance, latency and responsiveness.
>
> In comparison to the per-bdi interval/time, it's more a guarantee of
> smoother NFS writes. As the tests show in the original email, with
> the cost of a little more commits, it gains much better write
> throughput and latency.
>
> The NFS write queue is even a requirement, if we want to get
> reasonable good responsiveness. Without it, the 20% dirty limit may
> well be filled by NFS writeback/unstable pages. This is very bad for
> responsiveness. Let me quote contents of two old emails (with small
> fixes):
>
> : PG_writeback pages have been the biggest source of
> : latency issues in the various parts of the system.
> :
> : It's not uncommon for me to see filesystems sleep on PG_writeback
> : pages during heavy writeback, within some lock or transaction, which in
> : turn stall many tasks that try to do IO or merely dirty some page in
> : memory. Random writes are especially susceptible to such stalls. The
> : stable page feature also vastly increase the chances of stalls by
> : locking the writeback pages.
>
> : When there are N seconds worth of writeback pages, it may
> : take N/2 seconds on average for wait_on_page_writeback() to finish.
> : So the total time cost of running into a random writeback page and
> : waiting on it is also O(n^2):
>
> : E(PG_writeback waits) = P(hit PG_writeback) * E(wait on it)
>
> : That means we can hardly keep more than 1-second worth of writeback
> : pages w/o worrying about long waits on PG_writeback in various parts
> : of the kernel.
>
> : Page reclaim may also block on PG_writeback and/or PG_dirty pages. In
> : the case of direct reclaim, it means blocking random tasks that are
> : allocating memory in the system.
> :
> : PG_writeback pages are much worse than PG_dirty pages in that they are
> : not movable. This makes a big difference for high-order page allocations.
> : To make room for a 2MB huge page, vmscan has the option to migrate
> : PG_dirty pages, but for PG_writeback it has no better choices than to
> : wait for IO completion.
> :
> : The difficulty of THP allocation goes up *exponentially* with the
> : number of PG_writeback pages. Assume PG_writeback pages are randomly
> : distributed in the physical memory space. Then we have formula
> :
> : P(reclaimable for THP) = P(non-PG_writeback)^512
> :
> : That's the possibly for a contiguous range of 512 pages to be free of
> : PG_writeback, so that it's immediately reclaimable for use by
> : transparent huge page. This ruby script shows us the concrete numbers.
> :
> : irb> 1.upto(10) { |i| j=i/1000.0; printf "%.3f\t\t\t%.3f\n", j, (1-j)**512
> }
> :
> : P(hit PG_writeback) P(reclaimable for THP)
> : 0.001 0.599
> : 0.002 0.359
> : 0.003 0.215
> : 0.004 0.128
> : 0.005 0.077
> : 0.006 0.046
> : 0.007 0.027
> : 0.008 0.016
> : 0.009 0.010
> : 0.010 0.006
> :
> : The numbers show that when the PG_writeback pages go up from 0.1% to
> : 1% of system memory, the THP reclaim success ratio drops quickly from
> : 60% to 0.6%. It indicates that in order to use THP without constantly
> : running into stalls, the reasonable PG_writeback ratio is <= 0.1%.
> : Going beyond that threshold, it quickly becomes intolerable.
> :
> : That makes a limit of 256MB writeback pages for a mem=256GB system.
> : Looking at the real vmstat:nr_writeback numbers in dd write tests:
> :
> : JBOD-12SSD-thresh=8G/ext4-1dd-1-3.3.0/vmstat-end:nr_writeback 217009
> : JBOD-12SSD-thresh=8G/ext4-10dd-1-3.3.0/vmstat-end:nr_writeback 198335
> : JBOD-12SSD-thresh=8G/xfs-1dd-1-3.3.0/vmstat-end:nr_writeback 306026
> : JBOD-12SSD-thresh=8G/xfs-10dd-1-3.3.0/vmstat-end:nr_writeback 315099
> : JBOD-12SSD-thresh=8G/btrfs-1dd-1-3.3.0/vmstat-end:nr_writeback 1216058
> : JBOD-12SSD-thresh=8G/btrfs-10dd-1-3.3.0/vmstat-end:nr_writeback 895335
> :
> : Oops btrfs has 4GB writeback pages -- which asks for some bug fixing.
> : Even ext4's 800MB still looks way too high, but that's ~1s worth of
> : data per queue (or 130ms worth of data for the high performance Intel
> : SSD, which is perhaps in danger of queue underruns?). So this system
> : would require 512GB memory to comfortably run KVM instances with THP
> : support.
>
> The main concern on the NFS write wait queue, however, was that it
> might hurt performance for long fat network pipes with large
> bandwidth-delay products. If the pipe size can be properly estimated,
> we'll be able to set adequate queue size and remove the last obstacle
> of that patch.
>
> Thanks,
> Fengguang
>
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