Re: Integration of SCST in the mainstream Linux kernel

From: James Bottomley
Date: Mon Feb 04 2008 - 13:55:21 EST

On Mon, 2008-02-04 at 21:38 +0300, Vladislav Bolkhovitin wrote:
> James Bottomley wrote:
> > On Mon, 2008-02-04 at 20:56 +0300, Vladislav Bolkhovitin wrote:
> >
> >>James Bottomley wrote:
> >>
> >>>On Mon, 2008-02-04 at 20:16 +0300, Vladislav Bolkhovitin wrote:
> >>>
> >>>
> >>>>James Bottomley wrote:
> >>>>
> >>>>
> >>>>>>>>So, James, what is your opinion on the above? Or the overall SCSI target
> >>>>>>>>project simplicity doesn't matter much for you and you think it's fine
> >>>>>>>>to duplicate Linux page cache in the user space to keep the in-kernel
> >>>>>>>>part of the project as small as possible?
> >>>>>>>
> >>>>>>>
> >>>>>>>The answers were pretty much contained here
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>and here:
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>Weren't they?
> >>>>>>
> >>>>>>No, sorry, it doesn't look so for me. They are about performance, but
> >>>>>>I'm asking about the overall project's architecture, namely about one
> >>>>>>part of it: simplicity. Particularly, what do you think about
> >>>>>>duplicating Linux page cache in the user space to have zero-copy cached
> >>>>>>I/O? Or can you suggest another architectural solution for that problem
> >>>>>>in the STGT's approach?
> >>>>>
> >>>>>
> >>>>>Isn't that an advantage of a user space solution? It simply uses the
> >>>>>backing store of whatever device supplies the data. That means it takes
> >>>>>advantage of the existing mechanisms for caching.
> >>>>
> >>>>No, please reread this thread, especially this message:
> >>>> This is one of
> >>>>the advantages of the kernel space implementation. The user space
> >>>>implementation has to have data copied between the cache and user space
> >>>>buffer, but the kernel space one can use pages in the cache directly,
> >>>>without extra copy.
> >>>
> >>>
> >>>Well, you've said it thrice (the bellman cried) but that doesn't make it
> >>>true.
> >>>
> >>>The way a user space solution should work is to schedule mmapped I/O
> >>>from the backing store and then send this mmapped region off for target
> >>>I/O. For reads, the page gather will ensure that the pages are up to
> >>>date from the backing store to the cache before sending the I/O out.
> >>>For writes, You actually have to do a msync on the region to get the
> >>>data secured to the backing store.
> >>
> >>James, have you checked how fast is mmaped I/O if work size > size of
> >>RAM? It's several times slower comparing to buffered I/O. It was many
> >>times discussed in LKML and, seems, VM people consider it unavoidable.
> >
> >
> > Erm, but if you're using the case of work size > size of RAM, you'll
> > find buffered I/O won't help because you don't have the memory for
> > buffers either.
> James, just check and you will see, buffered I/O is a lot faster.

So in an out of memory situation the buffers you don't have are a lot
faster than the pages I don't have?

> >>So, using mmaped IO isn't an option for high performance. Plus, mmaped
> >>IO isn't an option for high reliability requirements, since it doesn't
> >>provide a practical way to handle I/O errors.
> >
> > I think you'll find it does ... the page gather returns -EFAULT if
> > there's an I/O error in the gathered region.
> Err, to whom return? If you try to read from a mmaped page, which can't
> be populated due to I/O error, you will get SIGBUS or SIGSEGV, I don't
> remember exactly. It's quite tricky to get back to the faulted command
> from the signal handler.
> Or do you mean mmap(MAP_POPULATE)/munmap() for each command? Do you
> think that such mapping/unmapping is good for performance?
> > msync does something
> > similar if there's a write failure.
> >
> >>>You also have to pull tricks with
> >>>the mmap region in the case of writes to prevent useless data being read
> >>>in from the backing store.
> >>
> >>Can you be more exact and specify what kind of tricks should be done for
> >>that?
> >
> > Actually, just avoid touching it seems to do the trick with a recent
> > kernel.
> Hmm, how can one write to an mmaped page and don't touch it?

I meant from user space ... the writes are done inside the kernel.

However, as Linus has pointed out, this discussion is getting a bit off
topic. There's no actual evidence that copy problems are causing any
performatince issues issues for STGT. In fact, there's evidence that
they're not for everything except IB networks.


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