Re: [PATCH v1 00/14] Multigenerational LRU

[Dave Hansen]

On 3/15/21 7:24 PM, Yu Zhao wrote:
> On Mon, Mar 15, 2021 at 11:00:06AM -0700, Dave Hansen wrote:
>> How bad does this scanning get in the worst case if there's a lot of
>> sharing?
> 
> Actually the improvement is larger when there is more sharing, i.e.,
> higher map_count larger improvement. Let's assume we have a shmem
> page mapped by two processes. To reclaim this page, we need to make
> sure neither PTE from the two sets of page tables has the accessed
> bit. The current page reclaim uses the rmap, i.e., rmap_walk_file().
> It first looks up the two VMAs (from the two processes mapping this
> shmem file) in the interval tree of this shmem file, then from each
> VMA, it goes through PGD/PUD/PMD to reach the PTE. The page can't be
> reclaimed if either of the PTEs has the accessed bit, therefore cost
> of the scanning is more than proportional to the number of accesses,
> when there is a lot sharing.
> 
> Why this series makes it better? We track the usage of page tables.
> Specifically, we work alongside switch_mm(): if one of the processes
> above hasn't be scheduled since the last scan, we don't need to scan
> its page tables. So the cost is roughly proportional to the number of
> accesses, regardless of how many processes. And instead of scanning
> pages one by one, we do it in large batches. However, page tables can
> be very sparse -- this is not a problem for the rmap because it knows
> exactly where the PTEs are (by vma_address()). We only know ranges (by
> vma->vm_start/vm_end). This is where the accessed bit on non-leaf
> PMDs can be of help.

That's an interesting argument.  *But*, this pivoted into describing an
optimization.  My takeaway from this is that large amounts of sharing
are probably only handled well if the processes doing the sharing are
not running constantly.

> But I guess you are wondering what downsides are. Well, we haven't
> seen any (yet). We do have page cache (non-shmem) heavy workloads,
> but not at a scale large enough to make any statistically meaningful
> observations. We are very interested in working with anybody who has
> page cache (non-shmem) heavy workloads and is willing to try out this
> series.

I would also be very interested to see some synthetic, worst-case
micros.  Maybe take a few thousand processes with very sparse page
tables that all map some shared memory.  They wake up long enough to
touch a few pages, then go back to sleep.

What happens if we do that?  I'm not saying this is a good workload or
that things must behave well, but I do find it interesting to watch the
worst case.

I think it would also be very worthwhile to include some research in
this series about why the kernel moved away from page table scanning.
What has changed?  Are the workloads we were concerned about way back
then not around any more?  Has faster I/O or larger memory sizes with a
stagnating page size changed something?

>> I'm kinda surprised by this, but my 16GB laptop has a lot more page
>> cache than I would have guessed:
>>
>>> Active(anon):    4065088 kB
>>> Inactive(anon):  3981928 kB
>>> Active(file):    2260580 kB
>>> Inactive(file):  3738096 kB
>>> AnonPages:       6624776 kB
>>> Mapped:           692036 kB
>>> Shmem:            776276 kB
>>
>> Most of it isn't mapped, but it's far from all being used for text.
> 
> We have categorized two groups:
>   1) average users that haven't experienced memory pressure since
>   their systems have booted. The booting process fills up page cache
>   with one-off file pages, and they remain until users experience
>   memory pressure. This can be confirmed by looking at those counters
>   of a freshly rebooted and idle system. My guess this is the case for
>   your laptop.

It's been up ~12 days.  There is ~10GB of data in swap, and there's been
a lot of scanning activity which I would associate with memory pressure:

> SwapCached:      1187596 kB
> SwapTotal:      51199996 kB
> SwapFree:       40419428 kB
...
> nr_vmscan_write 24900719
> nr_vmscan_immediate_reclaim 115535
> pgscan_kswapd 320831544
> pgscan_direct 23396383
> pgscan_direct_throttle 0
> pgscan_anon 127491077
> pgscan_file 216736850
> slabs_scanned 400469680
> compact_migrate_scanned 1092813949
> compact_free_scanned 4919523035
> compact_daemon_migrate_scanned 2372223
> compact_daemon_free_scanned 20989310
> unevictable_pgs_scanned 307388545


>   2) engineering users who store git repos and compile locally. They
>   complained about their browsers being janky because anon memory got
>   swapped even though their systems had a lot of stale file pages in
>   page cache, with the current page reclaim. They are what we consider
>   part of the page cache (non-shmem) heavy group.

Interesting.  You shouldn't have a shortage of folks like that among
kernel developers.