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Re: [dm-devel] [PATCH] Latest dm-userspace, with memory reclaim

From: Dan Smith <danms us ibm com>
Subject: Re: [dm-devel] [PATCH] Latest dm-userspace, with memory reclaim
Date: Sun, 10 Sep 2006 19:49:28 -0700

> FT> Yeah, I think that there is no performance difference under real
> FT> workloads (not synthetic benchmark workloads).
> So dbench is too synthetic?  Performance with guest domains has been
> quite good in my experience.

dbench isn't synthetic. I meant that we will see the performance
difference with a benchmark program designed to give I/Os hitting rmap
all the time.

> FT> Right, I should have said that the major advantage of ring buffer
> FT> is communication without system calls.
> That's definitely possible, but if the remap cache reduces the amount
> of communication with userspace, then you also save the context switch
> to map the data each time.  I know that blktap does not seem to suffer
> much of a performance hit here, so it may be lost in the noise of a
> domain switch.  What about native performance?  I intend for
> dm-userspace to be useful outside the realm of Xen :)

Well, I also have an example doing I/Os in user space in native
environments, the SCSI target framework (tgt).


> FT> I've not tried the origianl dm-userspace. 
> Then why do you claim that performance suffers because of the use of
> syscalls? 

You tried the design without rmap and implemented it due to poor
performance, didn't you?

> FT> You have all the equipment, so can you do a performance
> FT> comparison? 
> Yes, I can.

Great. Thanks in advance.

> FT> I guess that the results of Xen blktap and blkback drivers have
> FT> told us the expected performance differences.
> Is it not possible that blktap performs well for Xen because I/O
> latency is hidden by domain switches?

Maybe. But as I said, I have an example in native environments
too. But I like to wait for your performance comparison results than
expecting the performance now.

> I would think that on a single processor vanilla linux system that
> switching to userspace for every single map would not be ideal.

Yeah, not ideal. However, I think that adding more than 1,000-lines
complicated in-kernel code isn't ideal for unnoticeable performance
gain. We can simplify user-space programs too by deleting invalidating
rmap code.

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