Hello, On Fri 25-02-11 16:55:19, Jun'ichi Nomura wrote: > For range-cyclic writeback (e.g. kupdate), the writeback code sets > a continuation point of the next writeback to mapping->writeback_index. > > Current code sets the page next to the last written page. > I think it's intended for sequential writer. Not exactly. The code is meant so that background writeback gets to writing the end of a file which gets continuously dirtied (if we always started at the beginning, nr_to_write could always get to 0 before we reach end of the file). > However, in many cases, sequential writer is writing in the middle of the page > and it just redirties the last written page by continuing from that. > > So the next writeback should try to continue from the last written page, > not the next one. > (If it's clean because the writer was on the page boundary, > pagevec_lookup_tag just skips it. So no problem.) > > Otherwise, the last written page was left dirty until the writeback > wraps around. > > I.e. if the sequential dirtier has written on pagecache as '*'s below: > > |*******|*******|****---|-------|-------| ( |---| is a page ) > > then, writeback happens: > > |-------|-------|-------|-------|-------| > > and the dirtier continues: > > |-------|-------|----***|*******|*****--| > A B > > Next writeback should start from page A, not B. Yes, this is downside of our current scheme. Have you actually observed it in practice or is it mostly a theoretic concern? But as I'm thinking about it, it wouldn't harm our original aim to do what you propose and it can help this relatively common case. So I think it's a good idea. Fengguang, what do you think? Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
