2012/9/27, Jan Kara <jack@xxxxxxx>: > 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. Okay, I will make the patches as your opinion again. Thanks Jan and Wu ! > > 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
