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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 10:15:44 -0700

> FT> You said that the remap object mechanism improves the performance
> FT> by avoiding contacting user space for every requests. However, as
> FT> you see, Xen blktap (contacting user space for every requests) and
> FT> blkback (doing I/O in kernel) performances are comparable.
> 
> You may be right.  I suppose the page cache might serve to prevent
> repeated accesses to the same location on disk, thus making the remap
> cache mostly unnecessary?

Yeah, I think that there is no performance difference under real
workloads (not synthetic benchmark workloads).


> FT> dm-userspace's poor performance of user space access is due to
> FT> ioctl, an inefficient interface.
> 
> I do not use ioctl; I assume you're talking about the chardev
> read/write interface...
> 
> Do you have performance metrics that show it performs better without
> the remap cache?  I have gathered the following:
> 
> dbench to an LVM:  250-300 MB/s
> dbench to a dm-user cow, backed by LVM: 200-248 MB/s
> 
> Considering one is doing CoW and one is not, would you consider
> ~50MB/s too much of a hit?
> 
> FT> The blktap uses shared ring buffer between kernel and user space,
> FT> which enables us to batch multiple requests without data copies.
> 
> Well, the remap cache and the read/write interface do as much batching
> as possible to reduce communication with userspace.  Requests are
> copied to/from the kernel, which I suppose limits performance, but the
> request objects are rather small.

Right, I should have said that the major advantage of ring buffer is
communication without system calls.


> FT> Here's a patch to remove the remap object mechanism and add
> FT> replaced ioctl with ring buffer.
> 
> If performance does not suffer, I would love to remove the remap
> cache, as it is really ugly and complicated.  I suppose I have been
> under the assumption that it is absolutely necessary to achieve good
> performance, but if not, I will be the first to rip it out :)
> 
> A performance comparison with/without the remap cache would be
> appreciated. 

I'd like to say the same. :)

I've not tried the origianl dm-userspace. You have all the equipment,
so can you do a performance comparison? I guess that the results of
Xen blktap and blkback drivers have told us the expected performance
differences.


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