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Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new comman
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero |
Date: |
Thu, 11 Apr 2013 17:08:10 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4 |
Il 11/04/2013 16:57, Michael R. Hines ha scritto:
> 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,
Do you mean the TLB misses?
> 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().
is_dup_page() is memory-bound, not CPU-bound. Note that is_dup_page
only needs 1% of the bandwidth it scans (32 bytes for a cache line out
of 4096 bytes/page). Scanning 30 GB/s only requires reading 250 MB/s
from memory to the FSB.
> 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.
These are the real world scenarios that I was talking about. Do you
have profiles of these, with the latest QEMU code, that show
is_dup_page() to be expensive?
We could try prefetching the first cache line *of the next page* before
running is_dup_page. There's a lot of things to test before giving up
and inventing a new API.
Paolo
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, (continued)
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael S. Tsirkin, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Paolo Bonzini, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Message not available
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Paolo Bonzini, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael S. Tsirkin, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael S. Tsirkin, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero,
Paolo Bonzini <=
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Paolo Bonzini, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Paolo Bonzini, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Eric Blake, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Michael R. Hines, 2013/04/11
- Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero, Eric Blake, 2013/04/11
[Qemu-devel] [RFC PATCH RDMA support v1: 11/13] send pc.ram over RDMA, mrhines, 2013/04/10