linux-kernel - Re: [EXT] Re: [PATCH v2 00/22] add Object Storage Media Pool (mpool)

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Message-ID: <CAPcyv4iYOk3i0pPgXxDTy47BxWCXqqXS0J6mrY5o+1M_41XoAw@mail.gmail.com>
Date:   Tue, 20 Oct 2020 14:35:44 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Nabeel Meeramohideen Mohamed (nmeeramohide)" 
        <nmeeramohide@...ron.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "Steve Moyer (smoyer)" <smoyer@...ron.com>,
        "Greg Becker (gbecker)" <gbecker@...ron.com>,
        "Pierre Labat (plabat)" <plabat@...ron.com>,
        "John Groves (jgroves)" <jgroves@...ron.com>
Subject: Re: [EXT] Re: [PATCH v2 00/22] add Object Storage Media Pool (mpool)

On Mon, Oct 19, 2020 at 3:30 PM Nabeel Meeramohideen Mohamed
(nmeeramohide) <nmeeramohide@...ron.com> wrote:
>
> Hi Dan,
>
> On Friday, October 16, 2020 4:12 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > On Fri, Oct 16, 2020 at 2:59 PM Nabeel Meeramohideen Mohamed
> > (nmeeramohide) <nmeeramohide@...ron.com> wrote:
> > >
> > > On Thursday, October 15, 2020 2:03 AM, Christoph Hellwig
> > <hch@...radead.org> wrote:
> > > > I don't think this belongs into the kernel.  It is a classic case for
> > > > infrastructure that should be built in userspace.  If anything is
> > > > missing to implement it in userspace with equivalent performance we
> > > > need to improve out interfaces, although io_uring should cover pretty
> > > > much everything you need.
> > >
> > > Hi Christoph,
> > >
> > > We previously considered moving the mpool object store code to user-space.
> > > However, by implementing mpool as a device driver, we get several benefits
> > > in terms of scalability, performance, and functionality. In doing so, we relied
> > > only on standard interfaces and did not make any changes to the kernel.
> > >
> > > (1)  mpool's "mcache map" facility allows us to memory-map (and later unmap)
> > > a collection of logically related objects with a single system call. The objects in
> > > such a collection are created at different times, physically disparate, and may
> > > even reside on different media class volumes.
> > >
> > > For our HSE storage engine application, there are commonly 10's to 100's of
> > > objects in a given mcache map, and 75,000 total objects mapped at a given
> > time.
> > >
> > > Compared to memory-mapping objects individually, the mcache map facility
> > > scales well because it requires only a single system call and single
> > vm_area_struct
> > > to memory-map a complete collection of objects.
>
> > Why can't that be a batch of mmap calls on io_uring?
>
> Agreed, we could add the capability to invoke mmap via io_uring to help mitigate the
> system call overhead of memory-mapping individual objects, versus our mache map
> mechanism. However, there is still the scalability issue of having a vm_area_struct
> for each object (versus one for each mache map).
>
> We ran YCSB workload C in two different configurations -
> Config 1: memory-mapping each individual object
> Config 2: memory-mapping a collection of related objects using mcache map
>
> - Config 1 incurred ~3.3x additional kernel memory for the vm_area_struct slab -
> 24.8 MB (127188 objects) for config 1, versus 7.3 MB (37482 objects) for config 2.
>
> - Workload C exhibited around 10-25% better tail latencies (4-nines) for config 2,
> not sure if it's due the reduced complexity of searching VMAs during page faults.

So this gets to the meta question that is giving me pause on this
whole proposal:

    What does Linux get from merging mpool?

What you have above is a decent scalability bug report. That type of
pressure to meet new workload needs is how Linux interfaces evolve.
However, rather than evolve those interfaces mpool is a revolutionary
replacement that leaves the bugs intact for everyone that does not
switch over to mpool.

Consider io_uring as an example where the kernel resisted trends
towards userspace I/O engines and instead evolved a solution that
maintained kernel control while also achieving similar performance
levels.

The exercise is useful to identify places where Linux has
deficiencies, but wholesale replacing an entire I/O submission model
is a direction that leaves the old apis to rot.