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From: | Xiao Guangrong |
Subject: | Re: [Qemu-devel] [PATCH v3 09/10] migration: fix calculating xbzrle_counters.cache_miss_rate |
Date: | Wed, 8 Aug 2018 14:36:51 +0800 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 |
On 08/08/2018 02:05 PM, Peter Xu wrote:
On Tue, Aug 07, 2018 at 05:12:08PM +0800, address@hidden wrote:From: Xiao Guangrong <address@hidden> As Peter pointed out: | - xbzrle_counters.cache_miss is done in save_xbzrle_page(), so it's | per-guest-page granularity | | - RAMState.iterations is done for each ram_find_and_save_block(), so | it's per-host-page granularity | | An example is that when we migrate a 2M huge page in the guest, we | will only increase the RAMState.iterations by 1 (since | ram_find_and_save_block() will be called once), but we might increase | xbzrle_counters.cache_miss for 2M/4K=512 times (we'll call | save_xbzrle_page() that many times) if all the pages got cache miss. | Then IMHO the cache miss rate will be 512/1=51200% (while it should | actually be just 100% cache miss). And he also suggested as xbzrle_counters.cache_miss_rate is the only user of rs->iterations we can adapt it to count guest page numbers After that, rename 'iterations' to 'handle_pages' to better reflect its meaning Suggested-by: Peter Xu <address@hidden> Signed-off-by: Xiao Guangrong <address@hidden> --- migration/ram.c | 18 +++++++++--------- 1 file changed, 9 insertions(+), 9 deletions(-) diff --git a/migration/ram.c b/migration/ram.c index 09be01dca2..bd7c18d1f9 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -300,10 +300,10 @@ struct RAMState { uint64_t num_dirty_pages_period; /* xbzrle misses since the beginning of the period */ uint64_t xbzrle_cache_miss_prev; - /* number of iterations at the beginning of period */ - uint64_t iterations_prev; - /* Iterations since start */ - uint64_t iterations; + /* total handled pages at the beginning of period */ + uint64_t handle_pages_prev; + /* total handled pages since start */ + uint64_t handle_pages;The name is not that straightforward to me. I would think about "[guest|host]_page_count" or something better, or we just keep the old naming but with a better comment would be fine too.
The filed actually indicates total pages (target pages more precisely) handled during live migration. 'iterations' confuses us completely. It's target_page_count good to you?
/* number of dirty bits in the bitmap */ uint64_t migration_dirty_pages; /* last dirty_sync_count we have seen */ @@ -1587,19 +1587,19 @@ uint64_t ram_pagesize_summary(void)static void migration_update_rates(RAMState *rs, int64_t end_time){ - uint64_t iter_count = rs->iterations - rs->iterations_prev; + uint64_t page_count = rs->handle_pages - rs->handle_pages_prev;/* calculate period counters */ram_counters.dirty_pages_rate = rs->num_dirty_pages_period * 1000 / (end_time - rs->time_last_bitmap_sync);- if (!iter_count) {+ if (!page_count) { return; }if (migrate_use_xbzrle()) {xbzrle_counters.cache_miss_rate = (double)(xbzrle_counters.cache_miss - - rs->xbzrle_cache_miss_prev) / iter_count; + rs->xbzrle_cache_miss_prev) / page_count; rs->xbzrle_cache_miss_prev = xbzrle_counters.cache_miss; } } @@ -1657,7 +1657,7 @@ static void migration_bitmap_sync(RAMState *rs)migration_update_rates(rs, end_time); - rs->iterations_prev = rs->iterations;+ rs->handle_pages_prev = rs->handle_pages;/* reset period counters */rs->time_last_bitmap_sync = end_time; @@ -3209,7 +3209,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque) break; }- rs->iterations++;+ rs->handle_pages += pages;So it's still counting host pages, is this your intention to only change the name in the patch?
Hmm... the value returned by ram_find_and_save_block() isn't the total target pages posted out? /** * ram_find_and_save_block: finds a dirty page and sends it to f * * Called within an RCU critical section. * * Returns the number of pages written where zero means no dirty pages, * or negative on error ... * * On systems where host-page-size > target-page-size it will send all the * pages in a host page that are dirty. */
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