Re: Integration of SCST in the mainstream Linux kernel

[James Bottomley]

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
> >>>>>>>
> >>>>>>>http://marc.info/?l=linux-scsi&m=120164008302435
> >>>>>>>
> >>>>>>>and here:
> >>>>>>>
> >>>>>>>http://marc.info/?l=linux-scsi&m=120171067107293
> >>>>>>>
> >>>>>>>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: 
> >>>>http://marc.info/?l=linux-kernel&m=120169189504361&w=2. 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.

James


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