On Thu, Jun 04, 2020 at 09:51:10AM -0400, Zi Yan wrote: > On 4 Jun 2020, at 7:34, Matthew Wilcox wrote: > > On Thu, Jun 04, 2020 at 09:30:45AM +0530, Anshuman Khandual wrote: > >> +Quantifying Migration > >> +===================== > >> +Following events can be used to quantify page migration. > >> + > >> +- PGMIGRATE_SUCCESS > >> +- PGMIGRATE_FAIL > >> +- THP_MIGRATION_SUCCESS > >> +- THP_MIGRATION_FAILURE > >> + > >> +THP_MIGRATION_FAILURE in particular represents an event when a THP could not be > >> +migrated as a single entity following an allocation failure and ended up getting > >> +split into constituent normal pages before being retried. This event, along with > >> +PGMIGRATE_SUCCESS and PGMIGRATE_FAIL will help in quantifying and analyzing THP > >> +migration events including both success and failure cases. > > > > First, I'd suggest running this paragraph through 'fmt'. That way you > > don't have to care about line lengths. > > > > Second, this paragraph doesn't really explain what I need to know to > > understand the meaning of these numbers. When Linux attempts to migrate > > a THP, one of three things can happen: > > > > - It is migrated as a single THP > > - It is migrated, but had to be split > > - Migration fails > > > > How do I turn these four numbers into an understanding of how often each > > of those three situations happen? And why do we need four numbers to > > report three situations? > > > > Or is there something else that can happen? If so, I'd like that explained > > here too ;-) > > PGMIGRATE_SUCCESS and PGMIGRATE_FAIL record a combination of different events, > so it is not easy to interpret them. Let me try to explain them. Thanks! Very helpful explanation. > 1. migrating only base pages: PGMIGRATE_SUCCESS and PGMIGRATE_FAIL just mean > these base pages are migrated and fail to migrate respectively. > THP_MIGRATION_SUCCESS and THP_MIGRATION_FAILURE should be 0 in this case. > Simple. > > 2. migrating only THPs: > - PGMIGRATE_SUCCESS means THPs that are migrated and base pages > (from the split of THPs) that are migrated, > > - PGMIGRATE_FAIL means THPs that fail to migrate and base pages that fail to migrated. > > - THP_MIGRATION_SUCCESS means THPs that are migrated. > > - THP_MIGRATION_FAILURE means THPs that are split. > > So PGMIGRATE_SUCCESS - THP_MIGRATION_SUCCESS means the number of migrated base pages, > which are from the split of THPs. Are you sure about that? If I split a THP and each of those subpages migrates, won't I then see PGMIGRATE_SUCCESS increase by 512? > When it comes to analyze failed migration, PGMIGRATE_FAIL - THP_MIGRATION_FAILURE > means the number of pages that are failed to migrate, but we cannot tell how many > are base pages and how many are THPs. > > 3. migrating base pages and THP: > > The math should be very similar to the second case, except that > a) from PGMIGRATE_SUCCESS - THP_MIGRATION_SUCCESS, we cannot tell how many are pages begin > as base pages and how many are pages begin as THPs but become base pages after split; > b) from PGMIGRATE_FAIL - THP_MIGRATION_FAILURE, an additional case, > base pages that begin as base pages fail to migrate, is mixed into the number and we > cannot tell three cases apart. So why don't we just expose PGMIGRATE_SPLIT? That would be defined as the number of times we succeeded in migrating a THP but had to split it to succeed.
