Messages in this thread | | | Date | Fri, 19 Aug 2011 15:00:37 -0400 | From | Vivek Goyal <> | Subject | Re: [PATCH 5/5] writeback: IO-less balance_dirty_pages() |
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On Fri, Aug 19, 2011 at 10:54:06AM +0800, Wu Fengguang wrote: > Hi Vivek, > > > > + base_rate = bdi->dirty_ratelimit; > > > + pos_ratio = bdi_position_ratio(bdi, dirty_thresh, > > > + background_thresh, nr_dirty, > > > + bdi_thresh, bdi_dirty); > > > + if (unlikely(pos_ratio == 0)) { > > > + pause = MAX_PAUSE; > > > + goto pause; > > > } > > > + task_ratelimit = (u64)base_rate * > > > + pos_ratio >> RATELIMIT_CALC_SHIFT; > > > > Hi Fenguaang, > > > > I am little confused here. I see that you have already taken pos_ratio > > into account in bdi_update_dirty_ratelimit() and wondering why to take > > that into account again in balance_diry_pages(). > > > > We calculated the pos_rate and balanced_rate and adjusted the > > bdi->dirty_ratelimit accordingly in bdi_update_dirty_ratelimit(). > > Good question. There are some inter-dependencies in the calculation, > and the dependency chain is the opposite to the one in your mind: > balance_dirty_pages() used pos_ratio in the first place, so that > bdi_update_dirty_ratelimit() have to use pos_ratio in the calculation > of the balanced dirty rate, too. > > Let's return to how the balanced dirty rate is estimated. Please pay > special attention to the last paragraphs below the "......" line. > > Start by throttling each dd task at rate > > task_ratelimit = task_ratelimit_0 (1) > (any non-zero initial value is OK) > > After 200ms, we measured > > dirty_rate = # of pages dirtied by all dd's / 200ms > write_bw = # of pages written to the disk / 200ms > > For the aggressive dd dirtiers, the equality holds > > dirty_rate == N * task_rate > == N * task_ratelimit > == N * task_ratelimit_0 (2) > Or > task_ratelimit_0 = dirty_rate / N (3) > > Now we conclude that the balanced task ratelimit can be estimated by > > balanced_rate = task_ratelimit_0 * (write_bw / dirty_rate) (4) > > Because with (2) and (3), (4) yields the desired equality (1): > > balanced_rate == (dirty_rate / N) * (write_bw / dirty_rate) > == write_bw / N
Hi Fengguang,
Following is my understanding. Please correct me where I got it wrong.
Ok, I think I follow till this point. I think what you are saying is that following is our goal in a stable system.
task_ratelimit = write_bw/N (6)
So we measure the write_bw of a bdi over a period of time and use that as feedback loop to modify bdi->dirty_ratelimit which inturn modifies task_ratelimit and hence we achieve the balance. So we will start with some arbitrary task limit say task_ratelimit_0, and modify that limit over a period of time based on our feedback loop to achieve a balanced system. And following seems to be the formula. write_bw task_ratelimit = task_ratelimit_0 * ------- (7) dirty_rate Now I also understand that by using (2) and (3), you proved that how (7) will lead to (6) and that is our deisred goal.
> > ............................................................................. > > Now let's revisit (1). Since balance_dirty_pages() chooses to execute > the ratelimit > > task_ratelimit = task_ratelimit_0 > = dirty_ratelimit * pos_ratio (5) >
So balance_drity_pages() chose to take into account pos_ratio() also because for various reason like just taking into account only bandwidth variation as feedback was not sufficient. So we also took pos_ratio into account which in-trun is dependent on gloabal dirty pages and per bdi dirty_pages/rate.
So we refined the formula for calculating a tasks's effective rate over a period of time to following. write_bw task_ratelimit = task_ratelimit_0 * ------- * pos_ratio (9) dirty_rate Is my understanding right so far?
> Put (5) into (4), we get the final form used in > bdi_update_dirty_ratelimit() > > balanced_rate = (dirty_ratelimit * pos_ratio) * (write_bw / dirty_rate) > > So you really need to take (dirty_ratelimit * pos_ratio) as a single entity.
Now few questions.
- What is dirty_ratelimit in formula above?
- Is it wrong to understand the issue in following manner.
bdi->dirty_ratelimit is tracking write bandwidth variation on the bdi and effectively tracks write_bw/N.
bdi->dirty_ratelimit = write_bw/N
or
write_bw bdi->dirty_ratelimit = previous_bdi->dirty_ratelimit * ------------- (10) dirty_rate Hence a tasks's balanced rate from (9) and (10) is.
task_ratelimit = bdi->dirty_ratelimit * pos_ratio (11)
So my understanding about (10) and (11) is wrong? if no, then question comes that bdi->dirty_ratelimit is supposed to be keeping track of write bandwidth variations only. And in turn task ratelimit will be driven by both bandwidth varation as well as pos_ratio variation.
But you seem to be doing following.
bdi->dirty_ratelimit = adjust based on a cobination of bandwidth feedback and pos_ratio feedback.
task_ratelimit = bdi->dirty_ratelimit * pos_ratio (12)
So my question is that when task_ratelimit is finally being adjusted based on pos_ratio feedback, why bdi->dirty_ratelimit also needs to take that into account.
I know you have tried explaining it, but sorry, I did not get it. May be give it another shot in a layman's terms and I might understand it.
Thanks Vivek
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