Re: [Patch] mm: Increase pagevec size on large system

[Tim Chen]

On 6/26/20 8:47 PM, Andrew Morton wrote:
> On Sat, 27 Jun 2020 04:13:04 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
>> On Fri, Jun 26, 2020 at 02:23:03PM -0700, Tim Chen wrote:
>>> Enlarge the pagevec size to 31 to reduce LRU lock contention for
>>> large systems.
>>>
>>> The LRU lock contention is reduced from 8.9% of total CPU cycles
>>> to 2.2% of CPU cyles.  And the pmbench throughput increases
>>> from 88.8 Mpages/sec to 95.1 Mpages/sec.
>>
>> The downside here is that pagevecs are often stored on the stack (eg
>> truncate_inode_pages_range()) as well as being used for the LRU list.
>> On a 64-bit system, this increases the stack usage from 128 to 256 bytes
>> for this array.
>>
>> I wonder if we could do something where we transform the ones on the
>> stack to DECLARE_STACK_PAGEVEC(pvec), and similarly DECLARE_LRU_PAGEVEC
>> the ones used for the LRUs.  There's plenty of space in the header to
>> add an unsigned char sz, delete PAGEVEC_SIZE and make it an variable
>> length struct.
>>
>> Or maybe our stacks are now big enough that we just don't care.
>> What do you think?
> 
> And I wonder how useful CONFIG_NR_CPUS is for making this decision. 
> Presumably a lot of general-purpose kernel builds have CONFIG_NR_CPUS
> much larger than the actual number of CPUs.
> 
> I can't think of much of a fix for this, apart from making it larger on
> all kernels, Is there a downside to this?
> 

Thanks for Matthew and Andrew's feedbacks.

I am okay with Matthew's suggestion of keeping the stack pagevec size unchanged.
Andrew, do you have a preference?

I was assuming that for people who really care about saving the kernel memory
usage, they would make CONFIG_NR_CPUS small. I also have a hard time coming
up with a better scheme.

Otherwise, we will have to adjust the pagevec size when we actually 
found out how many CPUs we have brought online.  It seems like a lot
of added complexity for going that route.

Tim