Date:   Sat, 20 Nov 2021 22:28:35 +0100
From:   Nikolay Kichukov <nikolay@...um.net>
To:     Christian Schoenebeck <linux_oss@...debyte.com>,
        Dominique Martinet <asmadeus@...ewreck.org>
Cc:     v9fs-developer@...ts.sourceforge.net, netdev@...r.kernel.org,
        Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        Greg Kurz <groug@...d.org>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v3 6/7] 9p/trans_virtio: support larger msize values

On Sat, 2021-11-20 at 15:46 +0100, Christian Schoenebeck wrote:
> On Samstag, 20. November 2021 12:45:09 CET Dominique Martinet wrote:
> > (Thanks for restarting this thread, this had totally slipped out of
> > my
> > mind...)
> 
> Hi guys,
> 
> I have (more or less) silently been working on these patches on all
> ends in 
> the meantime. If desired I try to find some time next week to
> summarize 
> current status of this overall effort and what still needs to be done.

Great, I would be more than happy to test next version of these patches.

> 
> > Nikolay Kichukov wrote on Sat, Nov 20, 2021 at 12:20:35PM +0100:
> > > When the client mounts the share via virtio, requested msize is:
> > > 10485760 or 104857600
> > > 
> > > however the mount succeeds with:
> > > msize=507904 in the end as per the /proc filesystem. This is less
> > > than
> > > the previous maximum value.
> > 
> > (Not sure about this, I'll test these patches tomorrow, but since
> > something failed I'm not surprised you have less than what you could
> > have here: what do you get with a more reasonable value like 1M
> > first?)

It worked with 1MB, I can stick to this for the time being.

Are the kernel patches supposed to be included in the KVM host kernel or
would the guest kernel suffice?

> 
> The highest 'msize' value possible for me with this particular version
> of the 
> kernel patches is 4186212 (so slightly below 4MB).
> 
> Some benchmarks, linear reading a 12 GB file:
> 
> msize    average      notes
> 
> 8 kB     52.0 MB/s    default msize of Linux kernel <v5.15
> 128 kB   624.8 MB/s   default msize of Linux kernel >=v5.15
> 512 kB   1961 MB/s    current max. msize with any Linux kernel <=v5.15
> 1 MB     2551 MB/s    this msize would violate current virtio specs
> 2 MB     2521 MB/s    this msize would violate current virtio specs
> 4 MB     2628 MB/s    planned milestone
> 
> That does not mean it already makes sense to use these patches in this
> version 
> as is to increase overall performance yet, but the numbers already
> suggest 
> that increasing msize can improve performance on large sequential I/O.
> There 
> are still some things to do though to fix other use patterns from
> slowing down 
> with rising msize, which I will describe in a separate email.
> 
> I also have an experimental version of kernel patches and QEMU that
> goes as 
> high as msize=128MB. But that's a highly hacked version that copies
> buffers 
> between 9p client level and virtio level back and forth and with
> intentional 
> large buffer sizes on every 9p message type. This was solely meant as
> a stress 
> test, i.e. whether it was possible to go as high as virtio's
> theoretical 
> protocol limit in the first place, and whether it was stable. This
> stress test 
> was not about performance at all. And yes, I had it running with 128MB
> for 
> weeks without issues (except of being very slow of course due to hacks
> used).
> 
> > > In addition to the above, when the kernel on the guest boots and
> > > loads
> > > 9pfs support, the attached memory allocation failure trace is
> > > generated.
> > > 
> > > Is anyone else seeing similar and was anybody able to get msize
> > > set to
> > > 10MB via virtio protocol with these patches?
> > 
> > I don't think the kernel would ever allow this with the given code,
> > as
> > the "common" part of 9p is not smart enough to use scatter-gather
> > and
> > tries to do a big allocation (almost) the size of the msize:
> > 
> > ---
> >         clnt->fcall_cache =
> >                 kmem_cache_create_usercopy("9p-fcall-cache", clnt-
> > >msize,
> >                                            0, 0, P9_HDRSZ + 4,
> >                                            clnt->msize - (P9_HDRSZ +
> > 4),
> >                                            NULL);
> > 
> > ...
> >                 fc->sdata = kmem_cache_alloc(c->fcall_cache,
> > GFP_NOFS);
> > ---
> > So in practice, you will be capped at 2MB as that is the biggest the
> > slab will be able to hand over in a single chunk.
> 
> I did not encounter a 2MB limit here. But kmalloc() clearly has a 4MB
> limit, 
> so when trying an msize larger than 4MB it inevitably causes a memory 
> allocation error. In my tests this allocation error would always
> happen 
> immediately at mount time causing an instant kernel oops.
> 
> > It'll also make allocation failures quite likely as soon as the
> > system
> > has had some uptime (depending on your workload, look at
> > /proc/buddyinfo
> > if your machines normally have 2MB chunks available), so I would
> > really
> > not recommend running with buffers bigger than e.g. 512k on real
> > workloads -- it looks great on benchmarks, especially as it's on its
> > own
> > slab so as long as you're doing a lot of requests it will dish out
> > buffers fast, but it'll likely be unreliable over time.
> > (oh, and we allocate two of these per request...)
> > 
> > 
> > The next step to support large buffers really would be splitting
> > htat
> > buffer to:
> >  1/ not allocate huge buffers for small metadata ops, e.g. an open
> > call
> > certainly doesn't need to allocate 10MB of memory
> >  2/ support splitting the buffer in some scattergather array
> > 
> > Ideally we'd only allocate on an as-need basis, most of the protocol
> > calls bound how much data is supposed to come back and we know how
> > much
> > we want to send (it's a format string actually, but we can majorate
> > it
> > quite easily), so one would need to adjust all protocol calls to
> > pass
> > this info to p9_client_rpc/p9_client_zc_rpc so it only allocates
> > buffers
> > as required, if necessary in multiple reasonably-sized segments (I'd
> > love 2MB hugepages-backed folios...), and have all transports use
> > these
> > buffers.
> 
> It is not that bad in sense of pending work. One major thing that
> needs to be 
> done is to cap the majority of 9p message types to allocate only as
> much as 
> they need, which is for most message types <8k. Right now they always
> simply 
> kmalloc(msize), which hurts with increasing msize values. That task
> does not 
> need many changes though.
> 
> > I have a rough idea on how to do all this but honestly less than 0
> > time
> > for that, so happy to give advices or review any patch, but it's
> > going
> > to be a lot of work that stand in the way of really big IOs.
> 
> Reviews of the patches would actually help a lot to bring this overall
> effort 
> forward, but probably rather starting with the upcoming next version
> of the 
> kernel patches, not this old v3.
> 
> > > [    1.527981] 9p: Installing v9fs 9p2000 file system support
> > > [    1.528173] ------------[ cut here ]------------
> > > [    1.528174] WARNING: CPU: 1 PID: 791 at mm/page_alloc.c:5356
> > > __alloc_pages+0x1ed/0x290
> > This warning is exactly what I was saying about the allocation cap:
> > you've requested an allocation that was bigger than the max
> > __alloc_page
> > is willing to provide (MAX_ORDER, 11, so 2MB as I was saying)

Yes, this is no longer happening when I lowered the value of the msize
parameter.

> 
> Best regards,
> Christian Schoenebeck
> 
> 

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