Re: [PATCH] iomap: add allocation cache for iomap_dio
From: guzebing
Date: Mon Jan 05 2026 - 04:30:42 EST
在 2025/11/21 18:22, Christoph Hellwig 写道:
On Fri, Nov 21, 2025 at 05:00:52PM +0800, guzebing wrote:I’m sorry for the long delay in replying to your email due to some other matters. I hope you still remember this revision. First, thank you for your response.
From: guzebing <guzebing@xxxxxxxxxxxxx>
As implemented by the bio structure, we do the same thing on the
iomap-dio structure. Add a per-cpu cache for iomap_dio allocations,
enabling us to quickly recycle them instead of going through the slab
allocator.
By making such changes, we can reduce memory allocation on the direct
IO path, so that direct IO will not block due to insufficient system
memory. In addition, for direct IO, the read performance of io_uring
is improved by about 2.6%.
Have you checked how much of that you'd get by using a dedicated
slab cache that should also do per-cpu allocations? Note that even
if we had a dedicated per-cpu cache we'd probably still want that.
Yes, I try to use a dedicated kmem cache to allocate cache for iomap-dio structure. However, when system memory is sufficient, kmalloc and kmem cache deliver identical performance.
For direct I/O reads on the ext4 file system, the test command is:
./t/io_uring -p0 -d128 -b4096 -s32 -c32 -F1 -B1 -R1 -X1 -n1 -P1 /mnt/004.txt
The measured performance is:
kmalloc: 750K IOPS
kmem cache: 750K IOPS
per-CPU cache: 770K IOPS
For a mempool, we first allocate with kmalloc or kmem cache and finally fall back to a reserved cache—this is for reliability. It’s not a great fit for our high‑performance scenario.
Also any chance you could factor this into common code?
Additionally, the current need for frequent allocation/free (hundreds of thousands to millions of times per second) may be more suitable for the bio or dio structures; beyond those, I’m not sure whether similar scenarios exist.
If we were to extract a generic implementation solely for this, would it yield significant benefits? Do you have any good suggestions?
I’d appreciate your review.