Re: [Lsf] IO less throttling and cgroup aware writeback (Was: Re: Preliminary Agenda and Activities for LSF)

[Wu Fengguang <fengguang.wu@intel.com>]

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Sorry, attached is the "separate ACCOUNTING from THROTTLING" patch.

> It's very possible to throttle meta data READS/WRITES, as long as they
> can be attributed to the original task (assuming task oriented throttling
> instead of bio/request oriented).
> 
> The trick is to separate the concepts of THROTTLING and ACCOUNTING.
> You can ACCOUNT data and meta data reads/writes to the right task, and
> only to THROTTLE the task when it's doing data reads/writes.
> 
> FYI I played the same trick for balance_dirty_pages_ratelimited() for
> another reason: _accurate_ accounting of dirtied pages.
> 
> That trick should play well with most applications who do interleaved
> data and meta data reads/writes. For the special case of "find" who
> does pure meta data reads, we can still throttle it by playing another
> trick: to THROTTLE meta data reads/writes with a much higher threshold
> than that of data. So normal applications will be almost always be
> throttled at data accesses while "find" will be throttled at meta data
> accesses.
> 
> For a real example of how it works, you can check this patch (plus the
> attached one)
> 
> writeback: IO-less balance_dirty_pages()
> http://git.kernel.org/?p=linux/kernel/git/wfg/writeback.git;a=commitdiff;h=e0de5e9961eeb992f305e877c5ef944fcd7a4269;hp=992851d56d79d227beaba1e4dcc657cbcf815556
> 
> Where tsk->nr_dirtied does dirty ACCOUNTING and tsk->nr_dirtied_pause
> is the threshold for THROTTLING. When
> 
>         tsk->nr_dirtied > tsk->nr_dirtied_pause
> 
> The task will voluntarily enter balance_dirty_pages() for taking a
> nap (pause time will be proportional to tsk->nr_dirtied), and when
> finished, start a new account-and-throttle period by resetting
> tsk->nr_dirtied and possibly adjust tsk->nr_dirtied_pause for a more
> reasonable pause time at next sleep.
> 
> BTW, I'd like to advocate balance_dirty_pages() based IO controller :)
> 
> As you may have noticed, it's not all that hard: the main functions
> blkcg_update_bandwidth()/blkcg_update_dirty_ratelimit() can fit nicely
> in one screen!
> 
> writeback: async write IO controllers
> http://git.kernel.org/?p=linux/kernel/git/wfg/writeback.git;a=commitdiff;h=1a58ad99ce1f6a9df6618a4b92fa4859cc3e7e90;hp=5b6fcb3125ea52ff04a2fad27a51307842deb1a0
> 
> Thanks,
> Fengguang

[-- Attachment #2: writeback-accurate-task-dirtied.patch --]
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Subject: writeback: accurately account dirtied pages
Date: Thu Apr 14 07:52:37 CST 2011


Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/page-writeback.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- linux-next.orig/mm/page-writeback.c	2011-04-16 11:28:41.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-04-16 11:28:41.000000000 +0800
@@ -1352,8 +1352,6 @@ void balance_dirty_pages_ratelimited_nr(
 	if (!bdi_cap_account_dirty(bdi))
 		return;
 
-	current->nr_dirtied += nr_pages_dirtied;
-
 	if (dirty_exceeded_recently(bdi, MAX_PAUSE)) {
 		unsigned long max = current->nr_dirtied +
 						(128 >> (PAGE_SHIFT - 10));
@@ -1819,6 +1817,7 @@ void account_page_dirtied(struct page *p
 		__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
 		task_dirty_inc(current);
 		task_io_account_write(PAGE_CACHE_SIZE);
+		current->nr_dirtied++;
 	}
 }
 EXPORT_SYMBOL(account_page_dirtied);