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Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new comman

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero
Date: Thu, 11 Apr 2013 18:01:37 +0300

On Thu, Apr 11, 2013 at 10:57:26AM -0400, Michael R. Hines wrote:
> We have hardware already with front side bus speeds of 13 GB/s.
> We also already have 5 GB/s RDMA hardware, and we will likely
> have even faster RDMA hardware in the future.
> This analysis is not factoring into account the cycles it takes to
> map the pages before they are checked for duplicate bytes,
> regardless whether or not very little of the page is actually
> cached on the processor.
> This analysis is also not taking into account the possibility that the
> VM may be CPU-bound at the same time that QEMU is competing
> to execute is_dup_page().
> Thus, as you mentioned, a worst-case 5 GB/s memory bandwidth
> for is_dup_page() could be very easily reached given the right
> conditions - and we do have many workloads both HPC and Multi-tier
> which can easily cause QEMU's zero scanning performance to suffer.
> - Michael

Well, then you can make is_dup_page faster e.g. using the
pagemap trick as we discussed earlier.
Why does management need a "go fast" option? Just make it go fast...


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