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? Hi. Jan. 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 ? 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. Thanks. > > 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
