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Message-ID: <4D81D813.8060608@fusionio.com> Date: Thu, 17 Mar 2011 10:44:51 +0100 From: Jens Axboe <jaxboe@...ionio.com> To: Shaohua Li <shaohua.li@...el.com> CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "hch@...radead.org" <hch@...radead.org>, "jmoyer@...hat.com" <jmoyer@...hat.com>, Vivek Goyal <vgoyal@...hat.com> Subject: Re: [PATCH 04/10] block: initial patch for on-stack per-task plugging On 2011-03-17 04:19, Shaohua Li wrote: > On Thu, 2011-03-17 at 09:00 +0800, Shaohua Li wrote: >> On Thu, 2011-03-17 at 01:31 +0800, Vivek Goyal wrote: >>> On Wed, Mar 16, 2011 at 04:18:30PM +0800, Shaohua Li wrote: >>>> 2011/1/22 Jens Axboe <jaxboe@...ionio.com>: >>>>> Signed-off-by: Jens Axboe <jaxboe@...ionio.com> >>>>> --- >>>>> block/blk-core.c | 357 ++++++++++++++++++++++++++++++++------------ >>>>> block/elevator.c | 6 +- >>>>> include/linux/blk_types.h | 2 + >>>>> include/linux/blkdev.h | 30 ++++ >>>>> include/linux/elevator.h | 1 + >>>>> include/linux/sched.h | 6 + >>>>> kernel/exit.c | 1 + >>>>> kernel/fork.c | 3 + >>>>> kernel/sched.c | 11 ++- >>>>> 9 files changed, 317 insertions(+), 100 deletions(-) >>>>> >>>>> diff --git a/block/blk-core.c b/block/blk-core.c >>>>> index 960f12c..42dbfcc 100644 >>>>> --- a/block/blk-core.c >>>>> +++ b/block/blk-core.c >>>>> @@ -27,6 +27,7 @@ >>>>> #include <linux/writeback.h> >>>>> #include <linux/task_io_accounting_ops.h> >>>>> #include <linux/fault-inject.h> >>>>> +#include <linux/list_sort.h> >>>>> >>>>> #define CREATE_TRACE_POINTS >>>>> #include <trace/events/block.h> >>>>> @@ -213,7 +214,7 @@ static void blk_delay_work(struct work_struct *work) >>>>> >>>>> q = container_of(work, struct request_queue, delay_work.work); >>>>> spin_lock_irq(q->queue_lock); >>>>> - q->request_fn(q); >>>>> + __blk_run_queue(q); >>>>> spin_unlock_irq(q->queue_lock); >>>>> } >>>> Hi Jens, >>>> I have some questions about the per-task plugging. Since the request >>>> list is per-task, and each task delivers its requests at finish flush >>>> or schedule. But when one cpu delivers requests to global queue, other >>>> cpus don't know. This seems to have problem. For example: >>>> 1. get_request_wait() can only flush current task's request list, >>>> other cpus/tasks might still have a lot of requests, which aren't sent >>>> to request_queue. >>> >>> But very soon these requests will be sent to request queue as soon task >>> is either scheduled out or task explicitly flushes the plug? So we might >>> wait a bit longer but that might not matter in general, i guess. >> Yes, I understand there is just a bit delay. I don't know how severe it >> is, but this still could be a problem, especially for fast storage or >> random I/O. My current tests show slight regression (3% or so) with >> Jens's for 2.6.39/core branch. I'm still checking if it's caused by the >> per-task plug, but the per-task plug is highly suspected. >> >>>> your ioc-rq-alloc branch is for this, right? Will it >>>> be pushed to 2.6.39 too? I'm wondering if we should limit per-task >>>> queue length. If there are enough requests there, we force a flush >>>> plug. >>> >>> That's the idea jens had. But then came the question of maintaining >>> data structures per task per disk. That makes it complicated. >>> >>> Even if we move the accounting out of request queue and do it say at >>> bdi, ideally we shall to do per task per bdi accounting. >>> >>> Jens seemed to be suggesting that generally fluser threads are the >>> main cluprit for submitting large amount of IO. They are already per >>> bdi. So probably just maintain a per task limit for flusher threads. >> Yep, flusher is the main spot in my mind. We need call more flush plug >> for flusher thread. >> >>> I am not sure what happens to direct reclaim path, AIO deep queue >>> paths etc. >> direct reclaim path could build deep write queue too. It >> uses .writepage, currently there is no flush plug there. Maybe we need >> add flush plug in shrink_inactive_list too. >> >>>> 2. some APIs like blk_delay_work, which call __blk_run_queue() might >>>> not work. because other CPUs might not dispatch their requests to >>>> request queue. So __blk_run_queue will eventually find no requests, >>>> which might stall devices. >>>> Since one cpu doesn't know other cpus' request list, I'm wondering if >>>> there are other similar issues. >>> >>> So again in this case if queue is empty at the time of __blk_run_queue(), >>> then we will probably just experinece little more delay then intended >>> till some task flushes. But should not stall the system? >> not stall the system, but device stalls a little time. > Jens, > I need below patch to recover a ffsb fsync workload, which has about 30% > regression with stack plug. > I guess the reason is WRITE_SYNC_PLUG doesn't work now, so if a context > hasn't blk_plug, we lose previous plug (request merge). This suggests > all places we use WRITE_SYNC_PLUG before (for example, kjournald) should > have a blk_plug context. Good point, those should be auto-converted. I'll take this patch and double check the others. Thanks! Does it remove that performance regression completely? -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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