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Re: [Qemu-devel] [PATCHv2 7/9] migration: do not sent zero pages in bulk
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCHv2 7/9] migration: do not sent zero pages in bulk stage |
Date: |
Tue, 19 Mar 2013 11:36:48 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4 |
On 03/15/2013 09:50 AM, Peter Lieven wrote:
> during bulk stage of ram migration if a page is a
> zero page do not send it at all.
> the memory at the destination reads as zero anyway.
>
> even if there is an madvise with QEMU_MADV_DONTNEED
> at the target upon receival of a zero page I have observed
s/receival/receipt/
> that the target starts swapping if the memory is overcommitted.
> it seems that the pages are dropped asynchronously.
>
> Signed-off-by: Peter Lieven <address@hidden>
> ---
> arch_init.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
> +++ b/arch_init.c
> @@ -432,9 +432,11 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
> bytes_sent = -1;
> if (buffer_is_zero(p, TARGET_PAGE_SIZE)) {
> acct_info.dup_pages++;
> - bytes_sent = save_block_hdr(f, block, offset, cont,
> - RAM_SAVE_FLAG_COMPRESS);
> - qemu_put_byte(f, *p);
> + if (!ram_bulk_stage) {
> + bytes_sent = save_block_hdr(f, block, offset, cont,
> + RAM_SAVE_FLAG_COMPRESS);
> + qemu_put_byte(f, *p);
Hmm, in patch 5/9, why not 'qemu_put_byte(f, 0)' instead of spending
time dereferencing *p, since you already know the byte is 0?
> + }
> bytes_sent += 1;
Your accounting is now off. This increments bytes_sent even when
nothing was sent in the bulk phase. When touching this line, consider
using ++ instead of += 1.
> } else if (migrate_use_xbzrle()) {
> current_addr = block->offset + offset;
>
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- [Qemu-devel] [PATCHv2 3/9] buffer_is_zero: use vector optimizations if possible, (continued)
[Qemu-devel] [PATCHv2 7/9] migration: do not sent zero pages in bulk stage, Peter Lieven, 2013/03/15
- Re: [Qemu-devel] [PATCHv2 7/9] migration: do not sent zero pages in bulk stage,
Eric Blake <=
[Qemu-devel] [PATCHv2 5/9] migration: search for zero instead of dup pages, Peter Lieven, 2013/03/15
[Qemu-devel] [PATCHv2 1/9] move vector definitions to qemu-common.h, Peter Lieven, 2013/03/15
[Qemu-devel] [PATCHv2 6/9] migration: add an indicator for bulk state of ram migration, Peter Lieven, 2013/03/15