Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
On Fri 23-02-18 13:40:09, Robert Harris wrote: > If you are asking me to prove whether modifying the tuneable in the > manner above, thereby preferring compaction for more fragmented systems, > is successful then I can't answer now. I assume that the onus would > have been on Mel to show this at the time of the original commit. > However, I interpret his last comment on this patch as a request to > verify that changing the preference yields sane results. Yes, this is exactly were I was aiming... This might have been useful during the initial compaction implementation but I am not aware of any real users and I am also quite skeptical it is very much useful. I do realize that this is hand waving because I do not have any numbers at hands. The bottom line is that the users should care, really. The compaction should be as automatic as possible. We can argue about tuning for certain allocation orders and make the compaction more pro-active to provide lower latencies for those requests but deciding whether to reclaim or compact sounds like a too low level decision for admin to make and kind of unstable interface for different kernels as the implementation of the compaction changes over time. So I would really prefer to kill the tuning than try to "fix" it. -- Michal Hocko SUSE Labs
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
> On 23 Feb 2018, at 09:10, Michal Hocko wrote: > > On Mon 19-02-18 14:30:36, Robert Harris wrote: >> >> >>> On 19 Feb 2018, at 12:39, Michal Hocko wrote: >>> >>> On Mon 19-02-18 12:14:26, Robert Harris wrote: > On 19 Feb 2018, at 08:26, Michal Hocko wrote: > > On Sun 18-02-18 16:47:55, robert.m.har...@oracle.com wrote: >> From: "Robert M. Harris" >> >> __fragmentation_index() calculates a value used to determine whether >> compaction should be favoured over page reclaim in the event of >> allocation >> failure. The calculation itself is opaque and, on inspection, does not >> match its existing description. The function purports to return a value >> between 0 and 1000, representing units of 1/1000. Barring the case of a >> pathological shortfall of memory, the lower bound is instead 500. This >> is >> significant because it is the default value of sysctl_extfrag_threshold, >> i.e. the value below which compaction should be avoided in favour of page >> reclaim for costly pages. >> >> This patch implements and documents a modified version of the original >> expression that returns a value in the range 0 <= index < 1000. It >> amends >> the default value of sysctl_extfrag_threshold to preserve the existing >> behaviour. > > It is not really clear to me what is the actual problem you are trying > to solve by this patch. Is there any bug or are you just trying to > improve the current implementation to be more effective? There is not a significant bug. The first problem is that the mathematical expression in __fragmentation_index() is opaque, particularly given the lack of description in the comments or the original commit message. This patch provides such a description. Simply annotating the expression did not make sense since the formula doesn't work as advertised. The fragmentation index is described as being in the range 0 to 1000 but the bounds of the formula are instead 500 to 1000. This patch changes the formula so that its lower bound is 0. >>> >>> But why do we want to fix that in the first place? Why don't we simply >>> deprecate the tunable and remove it altogether? Who is relying on tuning >>> this option. Considering how it doesn't work as advertised and nobody >>> complaining I have that feeling that it is not really used in wild… >> >> I think it's a useful feature. Ignoring any contrived test case, there >> will always be a lower limit on the degree of fragmentation that can be >> achieved by compaction. If someone takes the trouble to measure this >> then it is entirely reasonable that he or she should be able to inhibit >> compaction for cases when fragmentation falls below some correspondingly >> sized threshold. > > Do you have any practical examples? Are you looking for proof that the existing feature is useful? It is possible today to induce compaction, observe a fragmentation index and then use the same index as a starting point for setting the tuneable. The fact that the actual range of reported indices is 500--1000 rather than the documented 0--1000 would have no practical effect on this approach. Therefore that fact that the feature doesn't work precisely as advertised does not mean that it is not useful. If you are asking me to prove whether modifying the tuneable in the manner above, thereby preferring compaction for more fragmented systems, is successful then I can't answer now. I assume that the onus would have been on Mel to show this at the time of the original commit. However, I interpret his last comment on this patch as a request to verify that changing the preference yields sane results. Robert Harris
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
On Mon 19-02-18 14:30:36, Robert Harris wrote: > > > > On 19 Feb 2018, at 12:39, Michal Hocko wrote: > > > > On Mon 19-02-18 12:14:26, Robert Harris wrote: > >> > >> > >>> On 19 Feb 2018, at 08:26, Michal Hocko wrote: > >>> > >>> On Sun 18-02-18 16:47:55, robert.m.har...@oracle.com wrote: > From: "Robert M. Harris" > > __fragmentation_index() calculates a value used to determine whether > compaction should be favoured over page reclaim in the event of > allocation > failure. The calculation itself is opaque and, on inspection, does not > match its existing description. The function purports to return a value > between 0 and 1000, representing units of 1/1000. Barring the case of a > pathological shortfall of memory, the lower bound is instead 500. This > is > significant because it is the default value of sysctl_extfrag_threshold, > i.e. the value below which compaction should be avoided in favour of page > reclaim for costly pages. > > This patch implements and documents a modified version of the original > expression that returns a value in the range 0 <= index < 1000. It > amends > the default value of sysctl_extfrag_threshold to preserve the existing > behaviour. > >>> > >>> It is not really clear to me what is the actual problem you are trying > >>> to solve by this patch. Is there any bug or are you just trying to > >>> improve the current implementation to be more effective? > >> > >> There is not a significant bug. > >> > >> The first problem is that the mathematical expression in > >> __fragmentation_index() is opaque, particularly given the lack of > >> description in the comments or the original commit message. This patch > >> provides such a description. > >> > >> Simply annotating the expression did not make sense since the formula > >> doesn't work as advertised. The fragmentation index is described as > >> being in the range 0 to 1000 but the bounds of the formula are instead > >> 500 to 1000. This patch changes the formula so that its lower bound is > >> 0. > > > > But why do we want to fix that in the first place? Why don't we simply > > deprecate the tunable and remove it altogether? Who is relying on tuning > > this option. Considering how it doesn't work as advertised and nobody > > complaining I have that feeling that it is not really used in wild… > > I think it's a useful feature. Ignoring any contrived test case, there > will always be a lower limit on the degree of fragmentation that can be > achieved by compaction. If someone takes the trouble to measure this > then it is entirely reasonable that he or she should be able to inhibit > compaction for cases when fragmentation falls below some correspondingly > sized threshold. Do you have any practical examples? -- Michal Hocko SUSE Labs
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
> On 19 Feb 2018, at 12:39, Michal Hocko wrote: > > On Mon 19-02-18 12:14:26, Robert Harris wrote: >> >> >>> On 19 Feb 2018, at 08:26, Michal Hocko wrote: >>> >>> On Sun 18-02-18 16:47:55, robert.m.har...@oracle.com wrote: From: "Robert M. Harris" __fragmentation_index() calculates a value used to determine whether compaction should be favoured over page reclaim in the event of allocation failure. The calculation itself is opaque and, on inspection, does not match its existing description. The function purports to return a value between 0 and 1000, representing units of 1/1000. Barring the case of a pathological shortfall of memory, the lower bound is instead 500. This is significant because it is the default value of sysctl_extfrag_threshold, i.e. the value below which compaction should be avoided in favour of page reclaim for costly pages. This patch implements and documents a modified version of the original expression that returns a value in the range 0 <= index < 1000. It amends the default value of sysctl_extfrag_threshold to preserve the existing behaviour. >>> >>> It is not really clear to me what is the actual problem you are trying >>> to solve by this patch. Is there any bug or are you just trying to >>> improve the current implementation to be more effective? >> >> There is not a significant bug. >> >> The first problem is that the mathematical expression in >> __fragmentation_index() is opaque, particularly given the lack of >> description in the comments or the original commit message. This patch >> provides such a description. >> >> Simply annotating the expression did not make sense since the formula >> doesn't work as advertised. The fragmentation index is described as >> being in the range 0 to 1000 but the bounds of the formula are instead >> 500 to 1000. This patch changes the formula so that its lower bound is >> 0. > > But why do we want to fix that in the first place? Why don't we simply > deprecate the tunable and remove it altogether? Who is relying on tuning > this option. Considering how it doesn't work as advertised and nobody > complaining I have that feeling that it is not really used in wild… I think it's a useful feature. Ignoring any contrived test case, there will always be a lower limit on the degree of fragmentation that can be achieved by compaction. If someone takes the trouble to measure this then it is entirely reasonable that he or she should be able to inhibit compaction for cases when fragmentation falls below some correspondingly sized threshold. I hope to improve upon the decison-making strategy in the allocator slow path but that is not a short term goal. The current patch is an improvement for the interim. Robert Harris
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
> On 19 Feb 2018, at 13:10, Mel Gorman wrote: > > On Mon, Feb 19, 2018 at 12:26:39PM +, Robert Harris wrote: >> >> >>> On 19 Feb 2018, at 09:47, Mel Gorman wrote: >>> >>> On Sun, Feb 18, 2018 at 04:47:55PM +, robert.m.har...@oracle.com wrote: From: "Robert M. Harris" __fragmentation_index() calculates a value used to determine whether compaction should be favoured over page reclaim in the event of allocation failure. The calculation itself is opaque and, on inspection, does not match its existing description. The function purports to return a value between 0 and 1000, representing units of 1/1000. Barring the case of a pathological shortfall of memory, the lower bound is instead 500. This is significant because it is the default value of sysctl_extfrag_threshold, i.e. the value below which compaction should be avoided in favour of page reclaim for costly pages. This patch implements and documents a modified version of the original expression that returns a value in the range 0 <= index < 1000. It amends the default value of sysctl_extfrag_threshold to preserve the existing behaviour. Signed-off-by: Robert M. Harris >>> >>> You have to update sysctl_extfrag_threshold as well for the new bounds. >> >> This patch makes its default value zero. >> > > Sorry, I'm clearly blind. > >>> It effectively makes it a no-op but it was a no-op already and adjusting >>> that default should be supported by data indicating it's safe. >> >> Would it be acceptable to demonstrate using tracing that in both the >> pre- and post-patch cases >> >> 1. compaction is attempted regardless of fragmentation index, >> excepting that >> >> 2. reclaim is preferred even for non-zero fragmentation during >> an extreme shortage of memory >> > > If you can demonstrate that for both reclaim-intensive and > compaction-intensive workloads then yes. Also include the reclaim and > compaction stats from /proc/vmstat and not just tracepoints to demonstrate > that reclaim doesn't get out of control and reclaim the world in > response to failed high-order allocations such as THP. Understood. Thanks. Robert Harris
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
On Mon, Feb 19, 2018 at 12:26:39PM +, Robert Harris wrote: > > > > On 19 Feb 2018, at 09:47, Mel Gorman wrote: > > > > On Sun, Feb 18, 2018 at 04:47:55PM +, robert.m.har...@oracle.com wrote: > >> From: "Robert M. Harris" > >> > >> __fragmentation_index() calculates a value used to determine whether > >> compaction should be favoured over page reclaim in the event of allocation > >> failure. The calculation itself is opaque and, on inspection, does not > >> match its existing description. The function purports to return a value > >> between 0 and 1000, representing units of 1/1000. Barring the case of a > >> pathological shortfall of memory, the lower bound is instead 500. This is > >> significant because it is the default value of sysctl_extfrag_threshold, > >> i.e. the value below which compaction should be avoided in favour of page > >> reclaim for costly pages. > >> > >> This patch implements and documents a modified version of the original > >> expression that returns a value in the range 0 <= index < 1000. It amends > >> the default value of sysctl_extfrag_threshold to preserve the existing > >> behaviour. > >> > >> Signed-off-by: Robert M. Harris > > > > You have to update sysctl_extfrag_threshold as well for the new bounds. > > This patch makes its default value zero. > Sorry, I'm clearly blind. > > It effectively makes it a no-op but it was a no-op already and adjusting > > that default should be supported by data indicating it's safe. > > Would it be acceptable to demonstrate using tracing that in both the > pre- and post-patch cases > > 1. compaction is attempted regardless of fragmentation index, > excepting that > > 2. reclaim is preferred even for non-zero fragmentation during > an extreme shortage of memory > If you can demonstrate that for both reclaim-intensive and compaction-intensive workloads then yes. Also include the reclaim and compaction stats from /proc/vmstat and not just tracepoints to demonstrate that reclaim doesn't get out of control and reclaim the world in response to failed high-order allocations such as THP. -- Mel Gorman SUSE Labs
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
On Mon 19-02-18 12:14:26, Robert Harris wrote: > > > > On 19 Feb 2018, at 08:26, Michal Hocko wrote: > > > > On Sun 18-02-18 16:47:55, robert.m.har...@oracle.com wrote: > >> From: "Robert M. Harris" > >> > >> __fragmentation_index() calculates a value used to determine whether > >> compaction should be favoured over page reclaim in the event of allocation > >> failure. The calculation itself is opaque and, on inspection, does not > >> match its existing description. The function purports to return a value > >> between 0 and 1000, representing units of 1/1000. Barring the case of a > >> pathological shortfall of memory, the lower bound is instead 500. This is > >> significant because it is the default value of sysctl_extfrag_threshold, > >> i.e. the value below which compaction should be avoided in favour of page > >> reclaim for costly pages. > >> > >> This patch implements and documents a modified version of the original > >> expression that returns a value in the range 0 <= index < 1000. It amends > >> the default value of sysctl_extfrag_threshold to preserve the existing > >> behaviour. > > > > It is not really clear to me what is the actual problem you are trying > > to solve by this patch. Is there any bug or are you just trying to > > improve the current implementation to be more effective? > > There is not a significant bug. > > The first problem is that the mathematical expression in > __fragmentation_index() is opaque, particularly given the lack of > description in the comments or the original commit message. This patch > provides such a description. > > Simply annotating the expression did not make sense since the formula > doesn't work as advertised. The fragmentation index is described as > being in the range 0 to 1000 but the bounds of the formula are instead > 500 to 1000. This patch changes the formula so that its lower bound is > 0. But why do we want to fix that in the first place? Why don't we simply deprecate the tunable and remove it altogether? Who is relying on tuning this option. Considering how it doesn't work as advertised and nobody complaining I have that feeling that it is not really used in wild... -- Michal Hocko SUSE Labs
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
> On 19 Feb 2018, at 09:47, Mel Gorman wrote: > > On Sun, Feb 18, 2018 at 04:47:55PM +, robert.m.har...@oracle.com wrote: >> From: "Robert M. Harris" >> >> __fragmentation_index() calculates a value used to determine whether >> compaction should be favoured over page reclaim in the event of allocation >> failure. The calculation itself is opaque and, on inspection, does not >> match its existing description. The function purports to return a value >> between 0 and 1000, representing units of 1/1000. Barring the case of a >> pathological shortfall of memory, the lower bound is instead 500. This is >> significant because it is the default value of sysctl_extfrag_threshold, >> i.e. the value below which compaction should be avoided in favour of page >> reclaim for costly pages. >> >> This patch implements and documents a modified version of the original >> expression that returns a value in the range 0 <= index < 1000. It amends >> the default value of sysctl_extfrag_threshold to preserve the existing >> behaviour. >> >> Signed-off-by: Robert M. Harris > > You have to update sysctl_extfrag_threshold as well for the new bounds. This patch makes its default value zero. > It effectively makes it a no-op but it was a no-op already and adjusting > that default should be supported by data indicating it's safe. Would it be acceptable to demonstrate using tracing that in both the pre- and post-patch cases 1. compaction is attempted regardless of fragmentation index, excepting that 2. reclaim is preferred even for non-zero fragmentation during an extreme shortage of memory ? Robert Harris
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
> On 19 Feb 2018, at 08:26, Michal Hocko wrote: > > On Sun 18-02-18 16:47:55, robert.m.har...@oracle.com wrote: >> From: "Robert M. Harris" >> >> __fragmentation_index() calculates a value used to determine whether >> compaction should be favoured over page reclaim in the event of allocation >> failure. The calculation itself is opaque and, on inspection, does not >> match its existing description. The function purports to return a value >> between 0 and 1000, representing units of 1/1000. Barring the case of a >> pathological shortfall of memory, the lower bound is instead 500. This is >> significant because it is the default value of sysctl_extfrag_threshold, >> i.e. the value below which compaction should be avoided in favour of page >> reclaim for costly pages. >> >> This patch implements and documents a modified version of the original >> expression that returns a value in the range 0 <= index < 1000. It amends >> the default value of sysctl_extfrag_threshold to preserve the existing >> behaviour. > > It is not really clear to me what is the actual problem you are trying > to solve by this patch. Is there any bug or are you just trying to > improve the current implementation to be more effective? There is not a significant bug. The first problem is that the mathematical expression in __fragmentation_index() is opaque, particularly given the lack of description in the comments or the original commit message. This patch provides such a description. Simply annotating the expression did not make sense since the formula doesn't work as advertised. The fragmentation index is described as being in the range 0 to 1000 but the bounds of the formula are instead 500 to 1000. This patch changes the formula so that its lower bound is 0. The fragmentation index is compared to the tuneable sysctl_extfrag_threshold, which defaults to 500. If the index is above this value then compaction is preferred over page reclaim in the event of allocation failure. Given the issue above, the index will almost always exceed the default threshold and compaction will occur even if there is low fragmentation. This patch changes the default value of the tuneable to 0, meaning that the existing behaviour will be unchanged. Changing sysctl_extfrag_threshold back to something non-zero in a future patch would effect the behaviour intended by the original code but would require more comprehensive testing since it would modify the kernel's performance under memory pressure. Robert Harris
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
On Sun, Feb 18, 2018 at 04:47:55PM +, robert.m.har...@oracle.com wrote: > From: "Robert M. Harris" > > __fragmentation_index() calculates a value used to determine whether > compaction should be favoured over page reclaim in the event of allocation > failure. The calculation itself is opaque and, on inspection, does not > match its existing description. The function purports to return a value > between 0 and 1000, representing units of 1/1000. Barring the case of a > pathological shortfall of memory, the lower bound is instead 500. This is > significant because it is the default value of sysctl_extfrag_threshold, > i.e. the value below which compaction should be avoided in favour of page > reclaim for costly pages. > > This patch implements and documents a modified version of the original > expression that returns a value in the range 0 <= index < 1000. It amends > the default value of sysctl_extfrag_threshold to preserve the existing > behaviour. > > Signed-off-by: Robert M. Harris You have to update sysctl_extfrag_threshold as well for the new bounds. It effectively makes it a no-op but it was a no-op already and adjusting that default should be supported by data indicating it's safe. -- Mel Gorman SUSE Labs
Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation_index()
On Sun 18-02-18 16:47:55, robert.m.har...@oracle.com wrote:
> From: "Robert M. Harris"
>
> __fragmentation_index() calculates a value used to determine whether
> compaction should be favoured over page reclaim in the event of allocation
> failure. The calculation itself is opaque and, on inspection, does not
> match its existing description. The function purports to return a value
> between 0 and 1000, representing units of 1/1000. Barring the case of a
> pathological shortfall of memory, the lower bound is instead 500. This is
> significant because it is the default value of sysctl_extfrag_threshold,
> i.e. the value below which compaction should be avoided in favour of page
> reclaim for costly pages.
>
> This patch implements and documents a modified version of the original
> expression that returns a value in the range 0 <= index < 1000. It amends
> the default value of sysctl_extfrag_threshold to preserve the existing
> behaviour.
It is not really clear to me what is the actual problem you are trying
to solve by this patch. Is there any bug or are you just trying to
improve the current implementation to be more effective?
> Signed-off-by: Robert M. Harris
> ---
> Documentation/sysctl/vm.txt | 2 +-
> mm/compaction.c | 2 +-
> mm/vmstat.c | 47
> +++--
> 3 files changed, 39 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index 5025ff9..384a78b 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -237,7 +237,7 @@ of memory, values towards 1000 imply failures are due to
> fragmentation and -1
> implies that the allocation will succeed as long as watermarks are met.
>
> The kernel will not compact memory in a zone if the
> -fragmentation index is <= extfrag_threshold. The default value is 500.
> +fragmentation index is <= extfrag_threshold. The default value is 0.
>
> ==
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 10cd757..9db6ef4 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1730,7 +1730,7 @@ static enum compact_result compact_zone_order(struct
> zone *zone, int order,
> return ret;
> }
>
> -int sysctl_extfrag_threshold = 500;
> +int sysctl_extfrag_threshold;
>
> /**
> * try_to_compact_pages - Direct compact to satisfy a high-order allocation
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 40b2db6..013f1af 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1044,15 +1044,22 @@ static void fill_contig_page_info(struct zone *zone,
> }
>
> /*
> - * A fragmentation index only makes sense if an allocation of a requested
> - * size would fail. If that is true, the fragmentation index indicates
> - * whether external fragmentation or a lack of memory was the problem.
> - * The value can be used to determine if page reclaim or compaction
> - * should be used
> + * If there is no block of at least the requested size, implying that an
> + * allocation would fail, then it might be possible to conjure one by
> + * compaction. As this is expensive it is reserved for those cases in which
> + * there is a relatively high degree of fragmentation. For low degrees, page
> + * reclaim is more appropriate since an allocation failure is more likely to
> be
> + * caused by a lack of memory.
> + *
> + * This function calculates an index in the range 0 to 1, expressed in units
> of
> + * 1/1000, indicating low and high fragmentation respectively. The special
> + * value of -1 indicates that free blocks of sufficient size are available
> and
> + * that an allocation should therefore succeed.
> */
> static int __fragmentation_index(unsigned int order, struct contig_page_info
> *info)
> {
> unsigned long requested = 1UL << order;
> + int result;
>
> if (WARN_ON_ONCE(order >= MAX_ORDER))
> return 0;
> @@ -1060,17 +1067,37 @@ static int __fragmentation_index(unsigned int order,
> struct contig_page_info *in
> if (!info->free_blocks_total)
> return 0;
>
> - /* Fragmentation index only makes sense when a request would fail */
> if (info->free_blocks_suitable)
> return -1000;
>
> /*
> - * Index is between 0 and 1 so return within 3 decimal places
> + * If the number of requested-size blocks that could be constructed if
> + * all free blocks were compacted is
> + *
> + * B = info->free_pages/requested
> + *
> + * then, conceptually, the number of fragments into which each
> + * requested-size block has been split is
> + *
> + * N = info->free_blocks_total/B
>*
> - * 0 => allocation would fail due to lack of memory
> - * 1 => allocation would fail due to fragmentation
> + * In the least and most fragmented cases all free memory resides on
> + * either the order - 1
