From: Dan Smith <danms@xxxxxxxxxx> Subject: Re: [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. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
