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Re: [Qemu-devel] [PATCHv2 7/9] migration: do not sent zero pages in bulk
From: |
Peter Lieven |
Subject: |
Re: [Qemu-devel] [PATCHv2 7/9] migration: do not sent zero pages in bulk stage |
Date: |
Tue, 19 Mar 2013 20:35:08 +0100 |
Am 19.03.2013 um 18:36 schrieb Eric Blake <address@hidden>:
> 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?
Good idea. Actually to avoid unnecessary checks in buffer_is_zero I think
I would like to change "if (buffer_is_zero(p, TARGET_PAGE_SIZE)) {..} " to
bool inline is_zero_page(uint8_t *p) {return buffer_find_nonzero_offset(p,
TARGET_PAGE_SIZE) == TARGET_PAGE_SIZE);}
...
if (is_zero_page(p)) {
…
}
also for better readability. For pages we know that they satisfy
can_use_buffer_find_nonzero_offset().
Peter
>
>> + }
>> 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
>
- Re: [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
[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