Messages in this thread | | | Subject | Re: [Question] Why PageReadahead is not migrated by migration code? | From | Yang Shi <> | Date | Thu, 13 Feb 2020 10:04:38 -0800 |
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On 2/13/20 9:33 AM, Matthew Wilcox wrote: > On Thu, Feb 13, 2020 at 09:06:58AM -0800, Yang Shi wrote: >> Recently we saw some PageReadahead related bugs, so I did a quick check >> about the use of PageReadahead. I just found the state is *not* migrated by >> migrate_page_states(). >> >> Since migrate_page() won't migrate writeback page, so if PageReadahead is >> set it should just mean PG_readahead rather than PG_reclaim. So, I didn't >> think of why it is not migrated. >> >> I dig into the history a little bit, but the change in migration code is too >> overwhelming. But, it looks PG_readahead was added after migration was >> introduced. Is it just a simple omission? > It's probably more that it just doesn't matter enough. If the Readahead > flag is missing on a page then the application will perform slightly worse > for a few pages as it ramps its readahead back up again. On the other > hand, you just migrated its pages to a different NUMA node, so chances > are there are bigger perofmrance problems happening at this moment anyway. > > I think we probably should migrate it, but I can understand why nobody's > noticed it before.
Thanks. I tend to agree the slight performance loss might be hidden by other things.
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