Stable pages, posible corner cases

Posted May 12, 2011 18:07 UTC (Thu) by davecb (subscriber, #1574)
In reply to: Stable pages by djwong
Parent article: Stable pages

Perhaps I'm misunderstanding, but won't a series of small sequential writes trigger wait-on-writeback? Or does this not apply to appending to a file-backed page?

In a previous life I was involved in the performance measurement of coalescing disk writes, and we found a very large number of sequential writes could be coalesced into single writes, and then adjacent blocks coalesced into larger singe writes. This paid off particularly well when a disk was being handed writes at or beyond it's capacity, by removing unneeded writes. I think I still have the graphs somewhere (;-))

I'll comment on the non-sequential case in a sec, after I look at my archive.

--dave

Stable pages, posible corner cases

Posted May 12, 2011 18:49 UTC (Thu) by jzbiciak (guest, #5246) [Link] (1 responses)

It seems like it should, but only if the page starts getting flushed to disk during the series of writes. Dirty pages don't get flushed to disk immediately unless there's memory pressure, too many dirty pages, they've been sitting around awhile, or you've asked them to be flushed. All those thresholds are defined throughout here:

http://lxr.free-electrons.com/source/mm/page-writeback.c

That's what makes it so hard (at least for me) to reason about what workloads would get hurt, since there's not a simple, immediate relationship between "application dirtied a page" and "page got scheduled for writeback." You need both of those things to happen *and* the application must subsequently try to dirty the page further before you hit the page-block.

I guess you could get some negative interactions more immediately if a 'write' call scheduled a writeback for part of a page, and then the app immediately resumed filling the rest of the page. Still, I don't think a write() syscall triggers an immediate writeback on most calls. Take a look at 'queue_io' around line 277:

http://lxr.free-electrons.com/source/fs/fs-writeback.c

Only the oldest dirtied pages get flushed, as I read that.

Stable pages, posible corner cases

Posted May 12, 2011 19:23 UTC (Thu) by davecb (subscriber, #1574) [Link]

Excellent, thanks! --dave