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Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting
From: |
Peter Xu |
Subject: |
Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting |
Date: |
Fri, 5 Mar 2021 09:22:50 -0500 |
Kunkun,
On Fri, Mar 05, 2021 at 03:50:34PM +0800, Kunkun Jiang wrote:
> When the host page is a huge page and something is sent in the
> current iteration, the migration_rate_limit() should be executed.
> If not, this function can be omitted to save time.
>
> Rename tmppages to pages_this_iteration to express its meaning
> more clearly.
>
> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
> ---
> migration/ram.c | 21 ++++++++++++++-------
> 1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/migration/ram.c b/migration/ram.c
> index a168da5cdd..9fc5b2997c 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1988,7 +1988,7 @@ static int ram_save_target_page(RAMState *rs,
> PageSearchStatus *pss,
> static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
> bool last_stage)
> {
> - int tmppages, pages = 0;
> + int pages = 0;
> size_t pagesize_bits =
> qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
> unsigned long start_page = pss->page;
> @@ -2000,21 +2000,28 @@ static int ram_save_host_page(RAMState *rs,
> PageSearchStatus *pss,
> }
>
> do {
> + int pages_this_iteration = 0;
> +
> /* Check if the page is dirty and send it if it is */
> if (!migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
> pss->page++;
> continue;
> }
>
> - tmppages = ram_save_target_page(rs, pss, last_stage);
> - if (tmppages < 0) {
> - return tmppages;
> + pages_this_iteration = ram_save_target_page(rs, pss, last_stage);
> + if (pages_this_iteration < 0) {
> + return pages_this_iteration;
> }
>
> - pages += tmppages;
> + pages += pages_this_iteration;
To me, both names are okay, it's just that the new name doesn't really provide
a lot more new information, while it's even longer...
Since you seem to prefer cleaning up tmppages, I'm actually thinking whether
it should be called as "pages" at all since ram_save_target_page() majorly only
returns either 1 if succeeded or <0 if error. There's only one very corner
case of xbzrle where it can return 0 in save_xbzrle_page():
if (encoded_len == 0) {
trace_save_xbzrle_page_skipping();
return 0;
}
I think it means the page didn't change at all, then I'm also wondering maybe
it can also return 1 showing one page migrated (though actually skipped!) which
should still be fine for the callers, e.g., ram_find_and_save_block() who will
finally check this "pages" value.
So I think _maybe_ that's a nicer cleanup to change that "return 0" to "return
1", then another patch to make the return value to be (1) return 0 if page
saved, or (2) return <0 if error. Then here in ram_save_host_page() tmppages
can be renamed to "ret" or "succeed".
> pss->page++;
> - /* Allow rate limiting to happen in the middle of huge pages */
> - migration_rate_limit();
> + /*
> + * Allow rate limiting to happen in the middle of huge pages if
> + * something is sent in the current iteration.
> + */
> + if (pagesize_bits > 1 && pages_this_iteration > 0) {
> + migration_rate_limit();
> + }
I don't know whether this matters either.. Two calls in there:
migration_update_counters(s, now);
qemu_file_rate_limit(s->to_dst_file);
migration_update_counters() is mostly a noop for 99.9% cases. Looks still
okay...
Thanks,
> } while ((pss->page & (pagesize_bits - 1)) &&
> offset_in_ramblock(pss->block,
> ((ram_addr_t)pss->page) <<
> TARGET_PAGE_BITS));
> --
> 2.23.0
>
--
Peter Xu
[PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting, Kunkun Jiang, 2021/03/05
- Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting,
Peter Xu <=
- Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting, Kunkun Jiang, 2021/03/08
- Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting, Peter Xu, 2021/03/08
- Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting, Kunkun Jiang, 2021/03/09
- Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting, Peter Xu, 2021/03/09
- Re: [PATCH v3 2/3] migration/ram: Reduce unnecessary rate limiting, Kunkun Jiang, 2021/03/09
[PATCH v3 3/3] migration/ram: Optimize ram_save_host_page(), Kunkun Jiang, 2021/03/05