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Message-ID: <aPJrmdeQY1QvbVdc@milan>
Date: Fri, 17 Oct 2025 18:15:21 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: Uladzislau Rezki <urezki@...il.com>,
Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] mm/vmalloc: request large order pages from buddy
allocator
On Thu, Oct 16, 2025 at 12:02:59PM -0700, Vishal Moola (Oracle) wrote:
> On Thu, Oct 16, 2025 at 10:42:04AM -0700, Vishal Moola (Oracle) wrote:
> > On Thu, Oct 16, 2025 at 06:12:36PM +0200, Uladzislau Rezki wrote:
> > > On Wed, Oct 15, 2025 at 02:28:49AM -0700, Vishal Moola (Oracle) wrote:
> > > > On Wed, Oct 15, 2025 at 04:56:42AM +0100, Matthew Wilcox wrote:
> > > > > On Tue, Oct 14, 2025 at 11:27:54AM -0700, Vishal Moola (Oracle) wrote:
> > > > > > Running 1000 iterations of allocations on a small 4GB system finds:
> > > > > >
> > > > > > 1000 2mb allocations:
> > > > > > [Baseline] [This patch]
> > > > > > real 46.310s real 34.380s
> > > > > > user 0.001s user 0.008s
> > > > > > sys 46.058s sys 34.152s
> > > > > >
> > > > > > 10000 200kb allocations:
> > > > > > [Baseline] [This patch]
> > > > > > real 56.104s real 43.946s
> > > > > > user 0.001s user 0.003s
> > > > > > sys 55.375s sys 43.259s
> > > > > >
> > > > > > 10000 20kb allocations:
> > > > > > [Baseline] [This patch]
> > > > > > real 0m8.438s real 0m9.160s
> > > > > > user 0m0.001s user 0m0.002s
> > > > > > sys 0m7.936s sys 0m8.671s
> > > > >
> > > > > I'd be more confident in the 20kB numbers if you'd done 10x more
> > > > > iterations.
> > > >
> > > > I actually ran my a number of times to mitigate the effects of possibly
> > > > too small sample sizes, so I do have that number for you too:
> > > >
> > > > [Baseline] [This patch]
> > > > real 1m28.119s real 1m32.630s
> > > > user 0m0.012s user 0m0.011s
> > > > sys 1m23.270s sys 1m28.529s
> > > >
> > > I have just had a look at performance figures of this patch. The test
> > > case is 16K allocation by one single thread, 1 000 000 loops, 10 run:
> > >
> > > sudo ./test_vmalloc.sh run_test_mask=1 nr_threads=1 nr_pages=4
> >
> > The reason I didn't use this test module is the same concern Matthew
> > brought up earlier about testing the PCP list rather than buddy
> > allocator. The test module allocates, then frees over and over again,
> > making it incredibly prone to reuse the pages over and over again.
> >
> > > BOX: AMD Milan, 256 CPUs, 512GB of memory
> > >
> > > # default 16K alloc
> > > [ 15.823704] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 955334 usec
> > > [ 17.751685] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1158739 usec
> > > [ 19.443759] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1016522 usec
> > > [ 21.035701] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 911381 usec
> > > [ 22.727688] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 987286 usec
> > > [ 24.199694] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 955112 usec
> > > [ 25.755675] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 926393 usec
> > > [ 27.355670] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 937875 usec
> > > [ 28.979671] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1006985 usec
> > > [ 30.531674] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 941088 usec
> > >
> > > # the patch 16K alloc
> > > [ 44.343380] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2296849 usec
> > > [ 47.171290] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2014678 usec
> > > [ 50.007258] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2094184 usec
> > > [ 52.651141] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1953046 usec
> > > [ 55.455089] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2209423 usec
> > > [ 57.943153] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1941747 usec
> > > [ 60.799043] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2038504 usec
> > > [ 63.299007] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1788588 usec
> > > [ 65.843011] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2137055 usec
> > > [ 68.647031] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2193022 usec
> > >
> > > 2X slower.
> > >
> > > perf-cycles, same test but on 64 CPUs:
> > >
> > > + 97.02% 0.13% [test_vmalloc] [k] fix_size_alloc_test
> > > - 82.11% 82.10% [kernel] [k] native_queued_spin_lock_slowpath
> > > 26.19% ret_from_fork_asm
> > > ret_from_fork
> > > - kthread
> > > - 25.96% test_func
> > > - fix_size_alloc_test
> > > - 23.49% __vmalloc_node_noprof
> > > - __vmalloc_node_range_noprof
> > > - 54.70% alloc_pages_noprof
> > > alloc_pages_mpol
> > > __alloc_frozen_pages_noprof
> > > get_page_from_freelist
> > > __rmqueue_pcplist
> > > - 5.58% __get_vm_area_node
> > > alloc_vmap_area
> > > - 20.54% vfree.part.0
> > > - 20.43% __free_frozen_pages
> > > free_frozen_page_commit
> > > free_pcppages_bulk
> > > _raw_spin_lock_irqsave
> > > native_queued_spin_lock_slowpath
> > > - 0.77% worker_thread
> > > - process_one_work
> > > - 0.76% vmstat_update
> > > refresh_cpu_vm_stats
> > > decay_pcp_high
> > > free_pcppages_bulk
> > > _raw_spin_lock_irqsave
> > > native_queued_spin_lock_slowpath
> > > + 76.57% 0.16% [kernel] [k] _raw_spin_lock_irqsave
> > > + 71.62% 0.00% [kernel] [k] __vmalloc_node_noprof
> > > + 71.61% 0.58% [kernel] [k] __vmalloc_node_range_noprof
> > > + 62.35% 0.06% [kernel] [k] alloc_pages_mpol
> > > + 62.27% 0.17% [kernel] [k] __alloc_frozen_pages_noprof
> > > + 62.20% 0.02% [kernel] [k] alloc_pages_noprof
> > > + 62.10% 0.05% [kernel] [k] get_page_from_freelist
> > > + 55.63% 0.19% [kernel] [k] __rmqueue_pcplist
> > > + 32.11% 0.00% [kernel] [k] ret_from_fork_asm
> > > + 32.11% 0.00% [kernel] [k] ret_from_fork
> > > + 32.11% 0.00% [kernel] [k] kthread
> > >
> > > I would say the bottle-neck is a page-allocator. It seems high-order
> > > allocations are not good for it.
>
> Ah also just took a closer look at this. I realize that you also did 16k
> allocations (which is at most order-2), so it may not be a good
> representation of high-order allocations either.
>
I agree. But then we should not optimize "small" orders and focus on
highest ones. Because of double degrade. I assume stress-ng fork test
would alos notice this.
> Plus that falls into the regression range I found that I detailed in
> response to Matthew elsewhere (I've copy pasted it here for reference)
>
> I ended up finding that allocating sizes <=20k had noticeable
> regressions, while [20k, 90k] was approximately the same, and >= 90k had
> improvements (getting more and more noticeable as size grows in
> magnitude).
>
Yes, i did 2-order allocations
# default
+ 35.87% 4.24% [kernel] [k] alloc_pages_bulk_noprof
+ 31.94% 0.88% [kernel] [k] vfree.part.0
- 27.38% 27.36% [kernel] [k] clear_page_rep
27.36% ret_from_fork_asm
ret_from_fork
kthread
test_func
fix_size_alloc_test
__vmalloc_node_noprof
__vmalloc_node_range_noprof
alloc_pages_bulk_noprof
clear_page_rep
# patch
+ 53.32% 1.12% [kernel] [k] get_page_from_freelist
+ 49.41% 0.71% [kernel] [k] prep_new_page
- 48.70% 48.64% [kernel] [k] clear_page_rep
48.64% ret_from_fork_asm
ret_from_fork
kthread
test_func
fix_size_alloc_test
__vmalloc_node_noprof
__vmalloc_node_range_noprof
alloc_pages_noprof
alloc_pages_mpol
__alloc_frozen_pages_noprof
get_page_from_freelist
prep_new_page
clear_page_rep
i noticed it is because of clear_page_rep() which with patch consumes
double in cycles.
Both versions should mostly go over pcp-cache, as far as i remember
order-2 is allowed to be cached.
I wounder why the patch gives x2 of cycles to clear_page_rep()...
--
Uladzislau Rezki
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