Re: Performance regression in scsi sequential throughput (iozone)due to "e084b - page-allocator: preserve PFN ordering when__GFP_COLD is set"

[Mel Gorman]

On Fri, Dec 18, 2009 at 02:38:15PM +0100, Christian Ehrhardt wrote:
> Christian Ehrhardt wrote:
>> Mel Gorman wrote:
>> [...]
>>>>  
>>>
>>> One way of finding out if cache hottness was the problem would be to  
>>> profile
>>> for cache misses and see if there are massive differences with and  
>>> without
>>> the patch. Is that an option?
>>>   
>> It is an option to verify things, but as mentioned above I would  
>> expect an increase amounts of consumed cycles per kb which I don't see.
>> I'll track caches anyway to be sure.
>>
>> My personal current assumption is that either there is some time lost  
>> from the read syscall until the A event blocktrace tracks or I'm wrong  
>> with my assumption about user processes and iozone runs "slower".
>>
>
> After I was able to verify that the time is lost above the block device  
> layer I ran further tests to check out where the additional latency due  
> to commit e084b occurs.
> With "strace -frT -e trace=open,close,read,write" I could isolate the  
> continuous stream of read calls done by the iozone child process.
> In those data two things were revealed:
> a) The time is lost between read and Block device enter (not e.g. due to  
> slow userspace execution or scheduling effects)
>     Time spent in userspace until the next read call is almost the same, 
> but the average time spent "inside" the read call increases  from 788µs 
> to 3278µs
> b) The time is lost in a few big peaks
>    The majority of read calls are ~500-750µs in both cases, but there is 
> a slight shift of approximately 8% that occur now as durations between 
> 15000 and 60000µs
>
> As mentioned before there is no significant increase of spent cpu cycles  
> - therefore I watched out how the read call could end up in sleeps  
> (latency but not used cycles).
> I found that there is a chance to run into "congestion_wait" in case  
> __alloc_pages_direct_reclaim() was able to free some pages with  
> try_to_free_pages() but doesn't get a page in the subsequent  
> get_page_from_freelist().

This is true although it was also the case before the patch.

> To get details about that I patched in some simple counters and  
> ktime_get based timings. The results are impressive and point exactly  
> towards the congestion_wait theory
>
> The following data is again taken with iozone 1 thread random read in a  
> very low memory environment, the counters were zeroed before starting 
> iozone
> Bad = the kernel just with commit e084b, good = kernel one commit before  
> e084b. The throughput in good case while taking this data was  
> approximately ~168% of that of the bad case.
>
>                                         GOOD     BAD
> perf_count_congestion_wait                245    1388
> perf_count_pages_direct_reclaim         10993    5365
> perf_count_failed_pages_direct_reclaim      0    1090
> Note - the data also shows that the average time spent in the  
> f_op->aio_read call issued by do_sync_read increases by 68.5% which is  
> directly inversely proportional to the average loss in throughput.
>
> This lets me conclude that patch e084b causes the system to run into  
> congestion_wait after the semi-failed call to 
> __alloc_pages_direct_reclaim.

You mention that the "GOOD" kernel is one commit before e084b but can
you test with just that patch reverted please? As the patch is only
about list manipulation, I'm failing to see how it can affect
congestion_wait().

> Unfortunately I don't understand memory management enough to find the  
> relation between e084b actually changing the order and position of freed  
> pages in the LRU list to __alloc_pages_direct_reclaim not getting pages  
> as effectively as before.

Off-hand, neither can I. I regret that I'm going off-line for the
Christmas very soon and I'll be in the middle of nowhere with no
internet connection or test machines in the interim. It's going to be
the new year before I get to look at this properly.

Even if this patch is not the real problem, your instrumentation patches
will help pin down the real problem. Thanks and sorry I'll be delayed in
getting back to you properly.

> Ideas and comments welcome!
>
> Kind regards,
> Christian
>
> P.S: attached are the three patches I used to get those new numbers
>
>
> -- 
>
> Grüsse / regards, Christian Ehrhardt
> IBM Linux Technology Center, Linux Performance on System z 
>

> Index: linux-2.6-git-2.6.31-bisect2/fs/read_write.c
> ===================================================================
> --- linux-2.6-git-2.6.31-bisect2.orig/fs/read_write.c
> +++ linux-2.6-git-2.6.31-bisect2/fs/read_write.c
> @@ -249,25 +249,37 @@ static void wait_on_retry_sync_kiocb(str
>  	__set_current_state(TASK_RUNNING);
>  }
>  
> +unsigned long perf_count_do_sync_read_calls_aio_read = 0;
> +unsigned long perf_tsum_do_sync_read_calls_aio_read = 0;
> +unsigned long perf_count_do_sync_read_calls_wait_on_sync_kiocb = 0;
> +unsigned long perf_tsum_do_sync_read_calls_wait_on_sync_kiocb = 0;
>  ssize_t do_sync_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos)
>  {
>  	struct iovec iov = { .iov_base = buf, .iov_len = len };
>  	struct kiocb kiocb;
>  	ssize_t ret;
> +	ktime_t tv64_pre;
>  
>  	init_sync_kiocb(&kiocb, filp);
>  	kiocb.ki_pos = *ppos;
>  	kiocb.ki_left = len;
>  
>  	for (;;) {
> +		perf_count_do_sync_read_calls_aio_read++;
> +		tv64_pre = ktime_get();
>  		ret = filp->f_op->aio_read(&kiocb, &iov, 1, kiocb.ki_pos);
> +		perf_tsum_do_sync_read_calls_aio_read += ktime_sub(ktime_get(),tv64_pre).tv64;
>  		if (ret != -EIOCBRETRY)
>  			break;
>  		wait_on_retry_sync_kiocb(&kiocb);
>  	}
>  
> -	if (-EIOCBQUEUED == ret)
> +	if (-EIOCBQUEUED == ret) {
> +		perf_count_do_sync_read_calls_wait_on_sync_kiocb++;
> +		tv64_pre = ktime_get();
>  		ret = wait_on_sync_kiocb(&kiocb);
> +		perf_tsum_do_sync_read_calls_wait_on_sync_kiocb += ktime_sub(ktime_get(),tv64_pre).tv64;
> +	}
>  	*ppos = kiocb.ki_pos;
>  	return ret;
>  }
> Index: linux-2.6-git-2.6.31-bisect2/kernel/sysctl.c
> ===================================================================
> --- linux-2.6-git-2.6.31-bisect2.orig/kernel/sysctl.c
> +++ linux-2.6-git-2.6.31-bisect2/kernel/sysctl.c
> @@ -257,6 +257,10 @@ extern unsigned long perf_count_pages_di
>  extern unsigned long perf_count_failed_pages_direct_reclaim;
>  extern unsigned long perf_count_do_generic_file_read_calls_page_cache_alloc_cold;
>  extern unsigned long perf_tsum_do_generic_file_read_calls_page_cache_alloc_cold;
> +extern unsigned long perf_count_do_sync_read_calls_aio_read;
> +extern unsigned long perf_tsum_do_sync_read_calls_aio_read;
> +extern unsigned long perf_count_do_sync_read_calls_wait_on_sync_kiocb;
> +extern unsigned long perf_tsum_do_sync_read_calls_wait_on_sync_kiocb;
>  static struct ctl_table perf_table[] = {
>  	{
>  		.ctl_name	= CTL_UNNUMBERED,
> @@ -297,6 +301,38 @@ static struct ctl_table perf_table[] = {
>  		.maxlen		= sizeof(unsigned long),
>  		.mode           = 0666,
>  		.proc_handler   = &proc_doulongvec_minmax,
> +	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname       = "perf_count_do_sync_read_calls_aio_read",
> +		.data           = &perf_count_do_sync_read_calls_aio_read,
> +		.maxlen		= sizeof(unsigned long),
> +		.mode           = 0666,
> +		.proc_handler   = &proc_doulongvec_minmax,
> +	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname       = "perf_tsum_do_sync_read_calls_aio_read",
> +		.data           = &perf_tsum_do_sync_read_calls_aio_read,
> +		.maxlen		= sizeof(unsigned long),
> +		.mode           = 0666,
> +		.proc_handler   = &proc_doulongvec_minmax,
> +	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname       = "perf_count_do_sync_read_calls_wait_on_sync_kiocb",
> +		.data           = &perf_count_do_sync_read_calls_wait_on_sync_kiocb,
> +		.maxlen		= sizeof(unsigned long),
> +		.mode           = 0666,
> +		.proc_handler   = &proc_doulongvec_minmax,
> +	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname       = "perf_tsum_do_sync_read_calls_wait_on_sync_kiocb",
> +		.data           = &perf_tsum_do_sync_read_calls_wait_on_sync_kiocb,
> +		.maxlen		= sizeof(unsigned long),
> +		.mode           = 0666,
> +		.proc_handler   = &proc_doulongvec_minmax,
>  	},
>  	{ .ctl_name = 0 }
>  };

> Index: linux-2.6-git-2.6.31-bisect2/include/linux/sysctl.h
> ===================================================================
> --- linux-2.6-git-2.6.31-bisect2.orig/include/linux/sysctl.h
> +++ linux-2.6-git-2.6.31-bisect2/include/linux/sysctl.h
> @@ -69,6 +69,7 @@ enum
>  	CTL_BUS=8,		/* Busses */
>  	CTL_ABI=9,		/* Binary emulation */
>  	CTL_CPU=10,		/* CPU stuff (speed scaling, etc) */
> +	CTL_PERF=11,		/* Performance counters and timer sums for debugging */
>  	CTL_ARLAN=254,		/* arlan wireless driver */
>  	CTL_S390DBF=5677,	/* s390 debug */
>  	CTL_SUNRPC=7249,	/* sunrpc debug */
> Index: linux-2.6-git-2.6.31-bisect2/kernel/sysctl.c
> ===================================================================
> --- linux-2.6-git-2.6.31-bisect2.orig/kernel/sysctl.c
> +++ linux-2.6-git-2.6.31-bisect2/kernel/sysctl.c
> @@ -177,6 +177,7 @@ static struct ctl_table_root sysctl_tabl
>  	.default_set.list = LIST_HEAD_INIT(root_table_header.ctl_entry),
>  };
>  
> +static struct ctl_table perf_table[];
>  static struct ctl_table kern_table[];
>  static struct ctl_table vm_table[];
>  static struct ctl_table fs_table[];
> @@ -230,6 +231,13 @@ static struct ctl_table root_table[] = {
>  		.mode		= 0555,
>  		.child		= dev_table,
>  	},
> +	{
> +		.ctl_name	= CTL_PERF,
> +		.procname	= "perf",
> +		.mode		= 0555,
> +		.child		= perf_table,
> +	},
> +
>  /*
>   * NOTE: do not add new entries to this table unless you have read
>   * Documentation/sysctl/ctl_unnumbered.txt
> @@ -244,6 +252,19 @@ static int min_wakeup_granularity_ns;
>  static int max_wakeup_granularity_ns = NSEC_PER_SEC;	/* 1 second */
>  #endif
>  
> +extern unsigned long perf_count_congestion_wait;
> +static struct ctl_table perf_table[] = {
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname       = "perf_count_congestion_wait",
> +		.data           = &perf_count_congestion_wait,
> +		.mode           = 0666,
> +		.maxlen		= sizeof(unsigned long),
> +		.proc_handler   = &proc_doulongvec_minmax,
> +	},
> +	{ .ctl_name = 0 }
> +};
> +
>  static struct ctl_table kern_table[] = {
>  #ifdef CONFIG_SCHED_DEBUG
>  	{
> Index: linux-2.6-git-2.6.31-bisect2/mm/backing-dev.c
> ===================================================================
> --- linux-2.6-git-2.6.31-bisect2.orig/mm/backing-dev.c
> +++ linux-2.6-git-2.6.31-bisect2/mm/backing-dev.c
> @@ -305,6 +305,7 @@ void set_bdi_congested(struct backing_de
>  }
>  EXPORT_SYMBOL(set_bdi_congested);
>  
> +unsigned long perf_count_congestion_wait = 0;
>  /**
>   * congestion_wait - wait for a backing_dev to become uncongested
>   * @sync: SYNC or ASYNC IO
> @@ -320,6 +321,7 @@ long congestion_wait(int sync, long time
>  	DEFINE_WAIT(wait);
>  	wait_queue_head_t *wqh = &congestion_wqh[sync];
>  
> +	perf_count_congestion_wait++;
>  	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
>  	ret = io_schedule_timeout(timeout);
>  	finish_wait(wqh, &wait);

> Index: linux-2.6-git-2.6.31-bisect2/kernel/sysctl.c
> ===================================================================
> --- linux-2.6-git-2.6.31-bisect2.orig/kernel/sysctl.c
> +++ linux-2.6-git-2.6.31-bisect2/kernel/sysctl.c
> @@ -253,6 +253,8 @@ static int max_wakeup_granularity_ns = N
>  #endif
>  
>  extern unsigned long perf_count_congestion_wait;
> +extern unsigned long perf_count_pages_direct_reclaim;
> +extern unsigned long perf_count_failed_pages_direct_reclaim;
>  static struct ctl_table perf_table[] = {
>  	{
>  		.ctl_name	= CTL_UNNUMBERED,
> @@ -262,6 +264,22 @@ static struct ctl_table perf_table[] = {
>  		.maxlen		= sizeof(unsigned long),
>  		.proc_handler   = &proc_doulongvec_minmax,
>  	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname       = "perf_count_pages_direct_reclaim",
> +		.data           = &perf_count_pages_direct_reclaim,
> +		.maxlen		= sizeof(unsigned long),
> +		.mode           = 0666,
> +		.proc_handler   = &proc_doulongvec_minmax,
> +	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname       = "perf_count_failed_pages_direct_reclaim",
> +		.data           = &perf_count_failed_pages_direct_reclaim,
> +		.maxlen		= sizeof(unsigned long),
> +		.mode           = 0666,
> +		.proc_handler   = &proc_doulongvec_minmax,
> +	},
>  	{ .ctl_name = 0 }
>  };
>  
> Index: linux-2.6-git-2.6.31-bisect2/mm/page_alloc.c
> ===================================================================
> --- linux-2.6-git-2.6.31-bisect2.orig/mm/page_alloc.c
> +++ linux-2.6-git-2.6.31-bisect2/mm/page_alloc.c
> @@ -1605,6 +1605,9 @@ out:
>  	return page;
>  }
>  
> +unsigned long perf_count_pages_direct_reclaim = 0;
> +unsigned long perf_count_failed_pages_direct_reclaim = 0;
> +
>  /* The really slow allocator path where we enter direct reclaim */
>  static inline struct page *
>  __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> @@ -1645,6 +1648,11 @@ __alloc_pages_direct_reclaim(gfp_t gfp_m
>  					zonelist, high_zoneidx,
>  					alloc_flags, preferred_zone,
>  					migratetype);
> +
> +	perf_count_pages_direct_reclaim++;
> +	if (!page)
> +		perf_count_failed_pages_direct_reclaim++;
> +
>  	return page;
>  }
>  


-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/