On 2020/9/23 23:18, Liu Hua wrote: > Hi Coly, > > Thanks for you reply! > > 在 2020/9/22 下午11:51, Coly Li 写道: >> On 2020/9/22 23:47, Liu Hua wrote: >>> Btree spliting and garbage collection process will drop >>> placeholders, which may cause cache miss collision. We can >>> insert nonoverlapping bkeys with write lock. It is helpful >>> for performance. >>> >> Could you please to explain more detais about, >> - How does "Btree spliting and garbage collection process will drop >> placeholders" happen? >> - And how does the consequence "cache miss collision" happen? > Cache miss process is as following: > > A. Insert placeholder with read lock > - lookup missed cache > - insert placeholder bkey > B. Read data from backing disk and write to cache > C. Insert real bkey with write lock > - check and fixup placeholder in bch_extent_insert_fixup. > - if bkey to be inserted does not match placeholder bkey. Inserting > will failed. > Bcache marks this type of failure as cache miss collision. we can > get this number > from /sys/block/bcacheX/bcache/cache/stats_total/cache_miss_collision > So a failed "c" process will lead the cache miss process fail, even if > we have data > in place but metadata failed. There are two types of reasons causing > this failure: > > 1. Two cache miss processes collide as following (two processes names > ABC and A'B'C') > A > A' > B > B' > C(will fail) > C'(will succeed) > 2. placeholder bkeys were dropped before "C". btree_mergesort called > from GC and > btree_split will drop "bad" bkeys including placeholder bkeys. > > For reason 2，since we hold write lock，if there is no overlaps with > existing bkeys. > we can insert bkeys safely. this is what this patch does. >> - Do you observe performance improvement? If yes, which part is improved >> and what is the performance number? > Can we treat this patch as a bugfix? We add a prefetch framework in > bcache and find 3% performace > improvement and reduction of collision in an order of one, in spark > word-count test with 1G ram > disk as cache. And I think it is helpful both for our situation and > original bcache. I can add a test based on original bcache. Copied. Give me some time to understand your change. Maybe more question will follow up. Thank you for the above detailed information :-) Coly Li
