On Thu 27-09-12 15:00:18, Namjae Jeon wrote: > 2012/9/27, Jan Kara <jack@xxxxxxx>: > > On Thu 27-09-12 00:56:02, Wu Fengguang wrote: > >> On Tue, Sep 25, 2012 at 12:23:06AM +0200, Jan Kara wrote: > >> > On Thu 20-09-12 16:44:22, Wu Fengguang wrote: > >> > > On Sun, Sep 16, 2012 at 08:25:42AM -0400, Namjae Jeon wrote: > >> > > > From: Namjae Jeon <namjae.jeon@xxxxxxxxxxx> > >> > > > > >> > > > This patch is based on suggestion by Wu Fengguang: > >> > > > https://lkml.org/lkml/2011/8/19/19 > >> > > > > >> > > > kernel has mechanism to do writeback as per dirty_ratio and > >> > > > dirty_background > >> > > > ratio. It also maintains per task dirty rate limit to keep balance > >> > > > of > >> > > > dirty pages at any given instance by doing bdi bandwidth > >> > > > estimation. > >> > > > > >> > > > Kernel also has max_ratio/min_ratio tunables to specify percentage > >> > > > of > >> > > > writecache to control per bdi dirty limits and task throttling. > >> > > > > >> > > > However, there might be a usecase where user wants a per bdi > >> > > > writeback tuning > >> > > > parameter to flush dirty data once per bdi dirty data reach a > >> > > > threshold > >> > > > especially at NFS server. > >> > > > > >> > > > dirty_background_centisecs provides an interface where user can > >> > > > tune > >> > > > background writeback start threshold using > >> > > > /sys/block/sda/bdi/dirty_background_centisecs > >> > > > > >> > > > dirty_background_centisecs is used alongwith average bdi write > >> > > > bandwidth > >> > > > estimation to start background writeback. > >> > The functionality you describe, i.e. start flushing bdi when there's > >> > reasonable amount of dirty data on it, looks sensible and useful. > >> > However > >> > I'm not so sure whether the interface you propose is the right one. > >> > Traditionally, we allow user to set amount of dirty data (either in > >> > bytes > >> > or percentage of memory) when background writeback should start. You > >> > propose setting the amount of data in centisecs-to-write. Why that > >> > difference? Also this interface ties our throughput estimation code > >> > (which > >> > is an implementation detail of current dirty throttling) with the > >> > userspace > >> > API. So we'd have to maintain the estimation code forever, possibly > >> > also > >> > face problems when we change the estimation code (and thus estimates in > >> > some cases) and users will complain that the values they set originally > >> > no > >> > longer work as they used to. > >> > >> Yes, that bandwidth estimation is not all that (and in theory cannot > >> be made) reliable which may be a surprise to the user. Which make the > >> interface flaky. > >> > >> > Also, as with each knob, there's a problem how to properly set its > >> > value? > >> > Most admins won't know about the knob and so won't touch it. Others > >> > might > >> > know about the knob but will have hard time figuring out what value > >> > should > >> > they set. So if there's a new knob, it should have a sensible initial > >> > value. And since this feature looks like a useful one, it shouldn't be > >> > zero. > >> > >> Agreed in principle. There seems be no reasonable defaults for the > >> centisecs-to-write interface, mainly due to its inaccurate nature, > >> especially the initial value may be wildly wrong on fresh system > >> bootup. This is also true for your proposed interfaces, see below. > >> > >> > So my personal preference would be to have bdi->dirty_background_ratio > >> > and > >> > bdi->dirty_background_bytes and start background writeback whenever > >> > one of global background limit and per-bdi background limit is exceeded. > >> > I > >> > think this interface will do the job as well and it's easier to maintain > >> > in > >> > future. > >> > >> bdi->dirty_background_ratio, if I understand its semantics right, is > >> unfortunately flaky in the same principle as centisecs-to-write, > >> because it relies on the (implicitly estimation of) writeout > >> proportions. The writeout proportions for each bdi starts with 0, > >> which is even worse than the 100MB/s initial value for > >> bdi->write_bandwidth and will trigger background writeback on the > >> first write. > > Well, I meant bdi->dirty_backround_ratio wouldn't use writeout proportion > > estimates at all. Limit would be > > dirtiable_memory * bdi->dirty_backround_ratio. > > > > After all we want to start writeout to bdi when we have enough pages to > > reasonably load the device for a while which has nothing to do with how > > much is written to this device as compared to other devices. > > > > OTOH I'm not particularly attached to this interface. Especially since on a > > lot of today's machines, 1% is rather big so people might often end up > > using dirty_background_bytes anyway. > > > >> bdi->dirty_background_bytes is, however, reliable, and gives users > >> total control. If we export this interface alone, I'd imagine users > >> who want to control centisecs-to-write could run a simple script to > >> periodically get the write bandwith value out of the existing bdi > >> interface and echo it into bdi->dirty_background_bytes. Which makes > >> simple yet good enough centisecs-to-write controlling. > >> > >> So what do you think about exporting a really dumb > >> bdi->dirty_background_bytes, which will effectively give smart users > >> the freedom to do smart control over per-bdi background writeback > >> threshold? The users are offered the freedom to do his own bandwidth > >> estimation and choose not to rely on the kernel estimation, which will > >> free us from the burden of maintaining a flaky interface as well. :) > > That's fine with me. Just it would be nice if we gave > > bdi->dirty_background_bytes some useful initial value. Maybe like > > dirtiable_memory * dirty_background_ratio? > Global dirty_background_bytes default value is zero that means > flushing is started based on dirty_background_ratio and dirtiable > memory. > Is it correct to set per bdi default dirty threshold > (bdi->dirty_background_bytes) equal to global dirty threshold - > dirtiable_memory * dirty_background_ratio ? Right, the default setting I proposed doesn't make a difference. And it's not obvious how to create one which is more meaningful. Pity. > In my opinion, default setting for per bdi-> dirty_background_bytes > should be zero to avoid any confusion and any change in default > writeback behaviour. OK, fine with me. 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
