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Re: [Qemu-block] [PATCH 3/9] mirror: optimize dirty bitmap filling in mi

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH 3/9] mirror: optimize dirty bitmap filling in mirror_run a bit
Date: Wed, 15 Jun 2016 06:25:25 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/15/2016 02:41 AM, Denis V. Lunev wrote:
> On 06/15/2016 05:36 AM, Eric Blake wrote:
>> On 06/14/2016 09:25 AM, Denis V. Lunev wrote:
>>> There is no need to scan allocation tables if we have mark_all_dirty
>>> flag
>>> set. Just mark it all dirty.

>>>       int ret, n;
>>>         end = s->bdev_length / BDRV_SECTOR_SIZE;
>>>   +    if (base == NULL && !bdrv_has_zero_init(target_bs)) {
>>> +        bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, end);
>> Won't work as written.  'end' is 64 bits, but bdrv_set_dirty_bitmap() is
>> limited to a 32-bit sector count.  Might be first worth updating
>> bdrv_set_dirty_bitmap() and friends to be byte-based rather than
>> sector-based (but still tracking a sector per bit, obviously), as well
>> as expand it to operate on 64-bit sizes rather than 32-bit.
> very nice catch! thank you

[meta-comment - your reply style is a bit hard to read. It's best to
include a blank line both before and after any text you write when
replying in context, as the extra spacing calls visual attention to your
reply rather than being part of a wall of text]

>> I'm also worried slightly that the existing code repeated things in a
>> loop, and therefore had pause points every iteration and could thus
>> remain responsive to an early cancel.  But doing the entire operation in
>> one chunk (assuming you fix bitmap code to handle a 64-bit size) may end
>> up running for so long without interruption that you lose the benefits
>> of an early interruption that you have by virtue of a 32-bit limit.
> I do not think that this should be worried actually. We just perform memset
> inside for not that big area (1 Tb disk will have 2 Mb dirty area
> bitmap) under
> default parameters.

Okay, so the real slowdown in the loop was the rest of the code
(checking backing status in bdrv_is_allocated_above() and not in
actually writing to the bitmap (bdrv_set_dirty_bitmap()).

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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