Re: [SPDK] BDEV-IO Lifecycle - Need your input.

[Walker, Benjamin <benjamin.walker at intel.com>]

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On Mon, 2018-07-09 at 16:18 +0000, Meneghini, John wrote:
> Hi Jim.
> 
> This is the patch series I believe Ben is proposing to replace Srikanth's:  ht
> tps://review.gerrithub.io/#/c/spdk/spdk/+/415860/
> 
> pick a21bbcf2 nvmf: Move data buffer pool to generic layer
> pick 09586e54 bdev: Add a zero copy I/O path
> pick 1cc9fe95 bdev: Make malloc bdev use the new zero copy mechanism
> pick 40a1f62b bdevperf: Use new zcopy API for reads
> pick eba6f35a bdev: Emulate zero copy support when necessary
> 
> Is this correct?


My patch series doesn't entirely replace Srikanth's set of patches. There are
two separate things necessary to implement zero copy. First is the
infrastructure within the bdev layer to make requests of a bdev module to obtain
 or release a suitable buffer for a zero copy operation. Second is actually
making the NVMe-oF target use that mechanism, which includes extending the
bdev_io lifetime. My patch series only addresses the first part.

Thanks,
Ben



> 
> /John
> 
> On 7/9/18, 11:31 AM, "Harris, James R" <james.r.harris(a)intel.com> wrote:
> 
>     Hi Sriram:
>     
>     Ben has some later patches in this series with some examples:
>     
>     https://review.gerrithub.io/#/c/spdk/spdk/+/386167/ - implements this zero
> copy API in the simple ‘malloc’ bdev module
>     https://review.gerrithub.io/#/c/spdk/spdk/+/416579/ - uses the zero copy
> API in the SPDK bdevperf utility
>     
>     -Jim
>     
>     
>     On 7/9/18, 3:34 AM, "Popuri, Sriram" <Sriram.Popuri(a)netapp.com> wrote:
>     
>         Sorry was not focusing on this change. Just give me a day or two to
> get back.
>         From a quick glance I didn't understand how zcopy start/end fits into
> a read/write life cycle. Is there an example on how zcopy start/end is
> consumed or someone can do give me a quick dump on how its envisioned to be
> used.
>         
>         Regards,
>         ~Sriram
>         
>         -----Original Message-----
>         From: Meneghini, John 
>         Sent: Friday, July 6, 2018 10:16 PM
>         To: Walker, Benjamin <benjamin.walker(a)intel.com>; Verkamp, Daniel <dan
> iel.verkamp(a)intel.com>; Harris, James R <james.r.harris(a)intel.com>; Kaligotla,
> Srikanth <Srikanth.Kaligotla(a)netapp.com>
>         Cc: raju.gottumukkala(a)broadcom.com; spdk(a)lists.01.org; Rodriguez,
> Edwin <Ed.Rodriguez(a)netapp.com>; Pai, Madhu <Madhusudan.Pai(a)netapp.com>; NGC-
> john.barnard-broadcom.com <john.barnard(a)broadcom.com>; Popuri, Sriram <Sriram.
> Popuri(a)netapp.com>
>         Subject: Re: BDEV-IO Lifecycle - Need your input.
>         
>         I'm adding Sriram to this thread.  Sriram is another NetApp engineer
> who is working on this stuff internally.
>         
>         /John
>         
>         On 7/6/18, 12:41 PM, "Walker, Benjamin" <benjamin.walker(a)intel.com>
> wrote:
>         
>             Hi Srikanth,
>             
>             I wanted to check in to see if you had any additional feedback on
> the zero copy
>             operations in the bdev layer here:
>             
>             https://review.gerrithub.io/#/c/spdk/spdk/+/386166/
>             
>             This does not address extending the lifetime of the bdev_io in the
> NVMe-oF
>             target, but I think this is looking like the right mechanism for
> the bdev layer.
>             
>             Thanks,
>             Ben
>             
>             On Thu, 2018-06-21 at 18:11 -0700, Harris, James R wrote:
>             > Thanks Srikanth.  Sounds like the spdk_bdev_io pool sizing along
> with
>             > spdk_bdev_queue_io_wait() meets your needs then regarding the
> spdk_bdev_io
>             > memory.
>             >  
>             > Regarding zcopy_start/zcopy_end – it looks like you’ve already
> added a bunch
>             > of comments to Ben’s patch on GerritHub.  For now I’d say let’s
> continue our
>             > discussion there.  I’ve responded to a couple of similar
> questions there and
>             > I’m sure Ben will have more replies tomorrow.
>             >  
>             > -Jim
>             >  
>             >  
>             > From: "Kaligotla, Srikanth" <Srikanth.Kaligotla(a)netapp.com>
>             > Date: Thursday, June 21, 2018 at 5:01 PM
>             > To: James Harris <james.r.harris(a)intel.com>, "Walker, Benjamin"
> <benjamin.walk
>             > er(a)intel.com>, Daniel Verkamp <daniel.verkamp(a)intel.com>
>             > Cc: "raju.gottumukkala(a)broadcom.com" <raju.gottumukkala(a)broadcom
> .com>,
>             > "Meneghini, John" <John.Meneghini(a)netapp.com>, "Rodriguez,
> Edwin" <Ed.Rodrigue
>             > z(a)netapp.com>, "Pai, Madhu" <Madhusudan.Pai(a)netapp.com>, "NGC-
> john.barnard-
>             > broadcom.com" <john.barnard(a)broadcom.com>, "spdk(a)lists.01.org" <
> spdk(a)lists.01.
>             > org>
>             > Subject: RE: BDEV-IO Lifecycle - Need your input.
>             >  
>             > Hi Jim,
>             >  
>             > I wish I joined the community meeting, I also missed the
>             > spdk_bdev_queue_io_wait().
>             >  
>             > So, there are 2 issues I am intending to solve,
>             >  
>             > 1.       We want to ensure that an instance of bdev-io is
> acquired prior to or
>             > along with I/O data buffer. This allows for better error
> handling when bdev_io
>             > pool is exhausted. Yes, we can solve it via sizing the pool
> right. The changes
>             > queue-io-wait also addresses the problem.
>             > 2.       Most importantly, by extending the life of bdev-io
> (Same life span as
>             > nvmf_request structure), abort use case and other use cases that
> involve
>             > cleaning up of IO data buffer are accomplished effectively. Let
> me elaborate;
>             > bdev-io is the fabric that connects the nvme command and
> operation with the
>             > backend. The driver context and data buffer context is stored in
> bdev-io. The
>             > freeing up of bdev_io resource is pushed to the end so the I/O
> cleanup can
>             > happen after controller has transmitted the data to the host. In
> absence of
>             > bdev_io, we will have end up adding more and more void context
> in request
>             > structure. Hence the push to extend the life of bdev_io.
>             >  
>             > I see Ben’s recent patch “zcopy_start” and “zcopy_end”; We can
> make that work
>             > as long as the bdev-io allocated/acquired stays till the end.
> One of the
>             > challenges I see with that patch is defining a callback for the
> I/O
>             > submission. For instance, zopy_start will allocate bdev_io and
> submits the I/O
>             > to the bdev device. Underlying implementation can be synchronous
> or can be
>             > asynchronous. The callback for this submission should check to
> see if it is a
>             > BUFFER-READY or PENDING and accordingly relay it back to
> transport. Next phase
>             > is actual I/O submission. Let’s say, it reuses the bdev-io
> obtained in ZCOPY-
>             > START, now the callback should determine if it is a success or
> failure.
>             > Finally when ZCOPY-END is invoked the supplied bdev_io will have
> all necessary
>             > data to release the WRITE buffer or unlock the read buffer based
> on the
>             > operation performed.
>             >  
>             > I hope I’m making sense. I guess, my effort is to extend the
> lifespan of bdev-
>             > io and let it continue to host the driver context and buffer
> context populated
>             > during the BEGIN phase.
>             >  
>             > Regards,
>             > Srikanth
>             >  
>             > From: Harris, James R <james.r.harris(a)intel.com> 
>             > Sent: Thursday, June 21, 2018 2:21 PM
>             > To: Kaligotla, Srikanth <Srikanth.Kaligotla(a)netapp.com>; Walker,
> Benjamin <ben
>             > jamin.walker(a)intel.com>; Verkamp, Daniel <daniel.verkamp(a)intel.c
> om>
>             > Cc: raju.gottumukkala(a)broadcom.com; Meneghini, John <John.Menegh
> ini(a)netapp.com
>             > >; Rodriguez, Edwin <Ed.Rodriguez(a)netapp.com>; Pai, Madhu <Madhu
> sudan.Pai(a)neta
>             > pp.com>; NGC-john.barnard-broadcom.com <john.barnard(a)broadcom.co
> m>; spdk(a)lists
>             > .01.org
>             > Subject: Re: BDEV-IO Lifecycle - Need your input.
>             >  
>             > Hi Srikanth,
>             >  
>             > Following up on this thread and the discussion in yesterday’s
> community
>             > meeting.
>             >  
>             > The recent spdk_bdev_queue_io_wait() changes allow an SPDK
> application to work
>             > better in general when the spdk_bdev_io buffer pool is
> exhausted.  We still
>             > need to make changes to the NVMe-oF target to use this new API
> but that should
>             > be done soon-ish.
>             >  
>             > With that in place, the spdk_bdev_io buffer pool itself can be
> configured up
>             > or down when the application starts.  Currently the default is
> 64K
>             > spdk_bdev_io buffers.  sizeof(struct spdk_bdev_io) == 216 plus
> the per-IO
>             > context size allocated for the bdev module.  This can be up to
> 192 bytes
>             > (virtio bdev module) but is likely much smaller for you
> depending on the
>             > context size for your ontap bdev module.
>             >  
>             > Let’s assume your per-IO context size is 64 bytes.  64K x (192 +
> 64) = 16MB.
>             >  
>             > I’m not sure how many spdk_bdev_io you need in flight at any
> given time.  64K
>             > seems like a lot but I’d be curious to hear your thoughts on
> this.  If this is
>             > the right number, then worst case, there would be about 16MB of
> DRAM that
>             > would sit unused if the NVMe-oF target in your system was not
> active.  Is that
>             > too burdensome for your application?
>             >  
>             > Thanks,
>             >  
>             > -Jim
>             >  
>             >  
>             >  
>             > From: "Kaligotla, Srikanth" <Srikanth.Kaligotla(a)netapp.com>
>             > Date: Wednesday, June 20, 2018 at 12:47 PM
>             > To: "Walker, Benjamin" <benjamin.walker(a)intel.com>, James Harris
> <james.r.harr
>             > is(a)intel.com>, Daniel Verkamp <daniel.verkamp(a)intel.com>
>             > Cc: "raju.gottumukkala(a)broadcom.com" <raju.gottumukkala(a)broadcom
> .com>,
>             > "Meneghini, John" <John.Meneghini(a)netapp.com>, "Rodriguez,
> Edwin" <Ed.Rodrigue
>             > z(a)netapp.com>, "Pai, Madhu" <Madhusudan.Pai(a)netapp.com>, "NGC-
> john.barnard-
>             > broadcom.com" <john.barnard(a)broadcom.com>, "spdk(a)lists.01.org" <
> spdk(a)lists.01.
>             > org>
>             > Subject: RE: BDEV-IO Lifecycle - Need your input.
>             >  
>             > Hello,
>             >  
>             > First revision of changes to extend the lifecycle of bdev_io are
> available for
>             > review. I would like to solicit your input on the proposed
> API/Code flow
>             > changes.
>             >  
>             > https://review.gerrithub.io/c/spdk/spdk/+/415860
>             >  
>             > Thanks,
>             > Srikanth
>             >  
>             >  
>             > From: "Kaligotla, Srikanth" <Srikanth.Kaligotla(a)netapp.com>
>             > Date: Friday, May 11, 2018 at 2:27 PM
>             > To: "Walker, Benjamin" <benjamin.walker(a)intel.com>, "Harris,
> James R" <james.r
>             > .harris(a)intel.com>
>             > Cc: "raju.gottumukkala(a)broadcom.com" <raju.gottumukkala(a)broadcom
> .com>,
>             > "Meneghini, John" <John.Meneghini(a)netapp.com>, "Rodriguez,
> Edwin" <Ed.Rodrigue
>             > z(a)netapp.com>, "Pai, Madhu" <Madhusudan.Pai(a)netapp.com>, "NGC-
> john.barnard-
>             > broadcom.com" <john.barnard(a)broadcom.com>, "spdk(a)lists.01.org" <
> spdk(a)lists.01.
>             > org>
>             > Subject: RE: BDEV-IO Lifecycle - Need your input.
>             >  
>             > CC: List
>             >  
>             > Hi Ben,
>             >  
>             > Your proposal to interface with the backend to acquire and
> release buffers is
>             > good. You have accurately stated that the challenge is in
> developing intuitive
>             > semantics. And that has been my struggle. To me, there are two
> problem
>             > statements;
>             >  
>             > 1.       It is expected that bdev_io pool is sized correctly so
> the call to
>             > get bdev_io succeeds. Failure to acquire bdev_io will result in
> DEVICE-ERROR.
>             > The transport is already capable of handling temporary memory
> failures by
>             > moving the request to PENDING-Queue. Hence the proposal to
> change the bdev_io
>             > lifecycle and perhaps connect it with the spdk_nvmf_request
> object. Thus all
>             > buffer needs are addressed at the beginning of I/O request.
>             > 2.       I/O Data buffers are sourced and managed by the
> backend. One of the
>             > challenges I see with your proposed interface is the lack of
> details like if
>             > resource is being acquired for a READ operation or WRITE
> operation. The
>             > handling is quite different in each case. Since bdev_io->type is
> overloaded
>             > the type of I/O operation is lost. I suppose one can cast the
> cb_arg
>             > (nvmf_request) and then proceed. Also, the bdev_io should be
> present to
>             > RELEASE the buffer. Zero copy semantics warrants that data
> buffer stays until
>             > controller-to-host has occurred. In other words, bdev_io lives
> till REQUEST
>             > come to COMPLETE state.
>             >  
>             > What are your thoughts in introducing spdk_bdev_init() and
> spdk_bdev_fini() as
>             > an alternative approach to extend the lifecyle of bdev_io and
> allow data
>             > buffer management via bdev fn_table ?
>             >  
>             > I hope I’m making sense…
>             >  
>             > Thanks,
>             > Srikanth
>             >  
>             > From: Walker, Benjamin <benjamin.walker(a)intel.com> 
>             > Sent: Friday, May 11, 2018 12:28 PM
>             > To: Harris, James R <james.r.harris(a)intel.com>; Kaligotla,
> Srikanth <Srikanth.
>             > Kaligotla(a)netapp.com>
>             > Cc: raju.gottumukkala(a)broadcom.com; Meneghini, John <John.Menegh
> ini(a)netapp.com
>             > >; Rodriguez, Edwin <Ed.Rodriguez(a)netapp.com>; Pai, Madhu <Madhu
> sudan.Pai(a)neta
>             > pp.com>; NGC-john.barnard-broadcom.com <john.barnard(a)broadcom.co
> m>
>             > Subject: Re: BDEV-IO Lifecycle - Need your input.
>             >  
>             > Hi Srikanth,
>             >  
>             > Yes - we'll need to introduce some way to acquire and release
> buffers from the
>             > bdev layer earlier on in the state machine that processes an
> NVMe-oF request.
>             > I've had this patch out for review for several months as a
> proposal for this
>             > scenario:
>             >  
>             > https://review.gerrithub.io/#/c/spdk/spdk/+/386166/
>             >  
>             > It doesn't pass the tests - it's just a proposal for the
> interface. 90% of the
>             > challenge here is in developing intuitive semantics.
>             >  
>             > Thanks,
>             > Ben
>             >  
>             > P.S. This is the kind of discussion that would fit perfectly on
> the mailing
>             > list.
>             >  
>             > On Wed, 2018-05-09 at 20:35 +0000, Kaligotla, Srikanth wrote:
>             > > Hi Ben, Hi James,
>             > >  
>             > > I would like to solicit opinions on the lifecycle for bdev-io
> resource
>             > > object. Attached is the image of RDMA state machine in its
> current
>             > > implementation. When the REQUEST enters the NEED-BUFFER state,
> the buffers
>             > > necessary for carrying out I/O operation are
> allocated/acquired from the
>             > > memory pool. An instance of BDEV-IO comes into existence after
> REQUEST
>             > > reaches READY-TO-EXECUTE state. The BDEV-IO is teared down as
> soon the
>             > > backend returns. From a BDEV perspective, BDEV-IO is simply a
> translation
>             > > unit that facilitates I/O buffers from the backend. The driver
> context
>             > > embedded within bdev_io holds great deal of information
> pertaining to the
>             > > I/O under execution. It assists in error handling,
> dereferencing the buffers
>             > > upon I/O completion and in abort handling. In summary, the
> bdev_io stays
>             > > alive until request has come to COMPLETE state. I’d like to
> hear peoples
>             > > thoughts in introducing the plumbing to acquire BDEV-IO
> resource in REQUEST-
>             > > NEED-BUFFER state and release it in REQUEST-COMPLETE state.  I
> will shortly
>             > > have patch available for review that introduces spdk_bdev_init
> and
>             > > spdk_bdev_fini which in turn invokes the corresponding bdev
> fn_table to
>             > >  initialize/cleanup.  
>             > >  
>             > > I wanted to use this email to communicate our intent and
> solicit your
>             > > feedback. We have a working implementation of the above
> proposal and prior
>             > > to pushing it upstream for review would like to hear your
> thoughts. These
>             > > proposed changes to upstream are a result of FC transport work
> in
>             > > collaboration with the Broadcom team who are also copied to
> this mail.
>             > > Myself and John will be at SPDK Dev conference and if required
> we can
>             > > elaborate further on this proposal.
>             > >  
>             > > Thanks,
>             > > Srikanth
>             > 
>             >  
>             
>         
>         
>     
>     
>