[dm-devel] [PATCH] Latest dm-userspace, with memory reclaim
FUJITA Tomonori
fujita.tomonori at lab.ntt.co.jp
Sun Sep 10 23:54:24 UTC 2006
From: Dan Smith <danms at 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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