I spent a bit of time to look at this and come up with some reasonable solution.
First, 128-byte cacheline is not dead, recently announced Marvell Octeon 10
have 128 byte cacheline.
In current code cacheline size defines both amount of data prefetch instruction
prefetches and
alignment of data in data structures needed to avoid false sharing.
So I think ideally we should have following:
- on x86:
- number of bytes prefetch instruction prefetches set to 64
- data structures should be aligned to 64 bytes
- due the fact that there is adjacent cacehline prefetcher on x86 it may be
worth
investigating if aligning to 128 brings some value
- on AArch64
- number of bytes prefetch instruction prefetches set to 64 or 128, based on
multiarch variant running
- data structures should be aligned to 128 bytes as that value prevents false
sharing for both 64 and 128 byte cacheline systems
Main problem is abuse of CLIB_PREFETCH() macro in our codebase.
Original idea of it was good, somebody wanted to provide macro which
transparently emits 1-4 prefetch
instructions based on data size recognising that there may be systems with
different cacheline size
Like:
CLIB_PREFETCH (p, sizeof (ip6_header_t), LOAD);
But reality is, most of the time we have:
CLIB_PREFETCH (p, CLIB_CACHE_LINE_BYTES, LOAD);
Where it is assumed that cacheline size is 64 and that just wasted resources on
system with 128-byte cacheline.
Also, most of places in our codebase are perfectly fine with whatever cacheline
size is, so I’m thinking about following:
1. set CLIB_CACHE_LINE_BYTES to 64 on x86 and 128 on ARM, that will make sure
false sharing is not happening
2. introduce CLIB_CACHE_PREFETCH_BYTES which can be set to different value for
different multiarch variant (64 for N1, 128 ThinderX2)
3. modify CLIB_PREFETCH macro to use CLIB_CACHE_PREFETCH_BYTES to emit proper
number of prefetch instructions for cases where data size is specified
4. take the stub and replace all `CLIB_PREFETCH (p, CLIB_CACHE_LINE_BYTES,
LOAD);` with `clib_prefetch_load (p);`.
There may be exceptions but those lines typically mean: "i want to prefetch
few (<=64) bytes at this address and i really don’t care what the cache line
size is”.
5. analyse remaining few cases where CLIB_PREFETCH() is used with size
specified by CLIB_CACHE_LINE_BYTES.
Thoughts?
—
Damjan
> On 06.07.2021., at 03:48, Lijian Zhang <lijian.zh...@arm.com> wrote:
>
> Thanks Damjan for your comments. Some replies in lines.
> <snip>
> Hi Lijian,
>
> It will be good to know if 128 byte cacheline is something ARM platforms will
> be using in the future or it is just historical.
> [Lijian] Existing ThunderX1 and OcteonTX2 CPUs are 128 byte cache-line. To my
> knowledge, there may be more CPUs with 128 byte cache-line in the future.
>
> Cacheline size problem is not just about prefetching, even bigger issue is
> false sharing, so we need to address both.
> [Lijian] Yes, there may be false-sharing issue when running VPP image with
> 64B definition on 128B cache-line CPUs. We will do some scalability testing
> with that case, and check the multi-core performance.
>
> Probably best solution is to have 2 VPP images, one for 128 and one for 64
> byte cacheline size.
> [Lijian] For native built image, that’s fine. But I’m not sure if it’s
> possible for cloud binaries installed via “apt-get install”.
>
> Going across the whole codebase and replacing prefetch macros is something we
> should definitely avoid.
> [Lijian] I got your concerns on large scope replacement. My concern is when
> CLIB_PREFETCH() is used to prefetch packet content into cache as below
> example, cache-line (CLIB_CACHE_LINE_BYTES) seems to be assumed as 64 bytes
> always.
> CLIB_PREFETCH (p2->data, 3 * CLIB_CACHE_LINE_BYTES, LOAD);
>
> —
> Damjan
>
>
> On 05.07.2021., at 07:28, Lijian Zhang <lijian.zh...@arm.com> wrote:
>
> Hi Damjan,
> I committed several patches to address some issues around cache-line
> definitions in VPP.
>
> Patch [1.1] is to resolve the build error [2] on 64Byte cache line Arm CPUs,
> e.g., ThunderX2, NeoverseN1, caused by the commit
> (https://gerrit.fd.io/r/c/vpp/+/32996, build: remove unused files and
> sections).
> It also supports building Arm generic image (with command of “make
> build-release”) with 128Byte cache line definition, and building native image
> with 64Byte cache line definition on some Arm CPUs, e.g., ThunderX2,
> NeoverseN1 (with command of “make build-release TARGET_PLATFORM=native”).
>
> Patch [1.5] is to set the default cache line definition in Arm generic image
> from 128Byte to 64Byte.
> Setting cache line definition to 128Byte for Arm generic image is required
> for ThunderX1 (with 128Byte physical cache line), which is also the build
> machine in FD.io lab. I’m thinking for setting 64Byte cache line definition
> in VPP for Arm image, which will affect ThunderX1 and OcteonTX2 CPUs. So it
> requires the confirmation by Marvell.
>
> Arm architecture CPUs have 128Byte or 64Byte physical designs. So no matter
> the cache line definition is 128Byte or 64Byte in VPP source code, the
> prefetch functions in generic image will not work properly on all Arm CPUs.
> Patches [1.2] [1.3] [1.4] are to resolve the issue.
>
> For example when running Arm generic image (cache-line-size is defined as
> 128B in Makefile for all Arm architectures) on 64Byte cache-line-size CPUs,
> e.g., Neoverse-N1, Ampere altra, ThunderX2.
>
> [3] shows the prefetch macro definitions in VPP. Using CLIB_PREFETCH(), you
> can prefetch data resides in multiple cache lines.
> [4] shows some usage examples of the prefetch macros in VPP. When running Arm
> generic image (128B cache-line-size definition) on 64B cache-line CPUs (N1SDP
> for example), 4.2, 4.3 and 4.4 have issues.
> • For 4.2, the input for size parameter is 68. On N1SDP with 64B
> cache-line-size, there should be two prefetch instructions executed, but due
> to 68 is less than CLIB_CACHE_LINE_BYTES (128Byte definition in VPP), only
> the first prefetch instruction is executed.
> • For 4.3, if sizeof (ip0[0]) equals 68 or any other values larger than
> 64B, there will be the same issue as 4.2.
> • For 4.4, the code is trying to prefetch the first 128B of packet
> content. It assumes CLIB_CACHE_LINE_BYTES is 64B always. In Arm generic
> image, the input for size parameter is 256B, which will execute prefetches on
> unexpected cache-lines (expected prefetches on 64B-0 and 64B-1, but actually
> on B64-0 and B64-2) .
> Packet content: [64B-0][64B-1][64B-2][64B-3]
>
> Our proposal is introduce a macro CLIB_N_CACHELINE_BYTES via VPP multi-arch
> feature (check patch [1.2]), to reflect the runtime CPU cache-line-size in
> Arm generic image, so that the prefetch instructions can be executed
> correctly.
> Then for 4.4, we will need to modify the parameter for size, from
> 2*CLIB_CACHE_LINE_BYTES to 128B, to reflect the actual intention.
>
> Some additional macros [1.3] can be added for users to do prefetch based on
> number of cache-lines, besides number of bytes.
>
> Could you please suggest on the issue and proposal?
>
> [1]. Patches
> [1.1] build: support 128B/64B cache line size in Arm image,
> https://gerrit.fd.io/r/c/vpp/+/32968/2
> [1.2] vppinfra: refactor prefetch macro,
> https://gerrit.fd.io/r/c/vpp/+/32969/3
> [1.3] vppinfra: fix functions to prefetch single line,
> https://gerrit.fd.io/r/c/vpp/+/32970/2
> [1.4] misc: correct prefetch macro usage,
> https://gerrit.fd.io/r/c/vpp/+/32971/3
> [1.5] build: set 64B cache line size in Arm image,
> https://gerrit.fd.io/r/c/vpp/+/32972/2
>
> [2]. Error message
> src/plugins/dpdk/device/init.c:1916:3: error: static_assert failed due to
> requirement '128 == 1 << 6' "DPDK RTE CACHE LINE SIZE does not match with
> 1<<CLIB_LOG2_CACHE_LINE_BYTES"
> STATIC_ASSERT (RTE_CACHE_LINE_SIZE == 1 << CLIB_LOG2_CACHE_LINE_BYTES,
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /home/lijian/tasks/plsremove/src/vppinfra/error_bootstrap.h:111:34: note:
> expanded from macro 'STATIC_ASSERT'
> #define STATIC_ASSERT(truth,...) _Static_assert(truth, __VA_ARGS__)
> ^ ~~~~~
> 1 error generated.
>
> [3]. Prefetch macro definitions in VPP source code.
> ‘’’
> #define _CLIB_PREFETCH(n,size,type)
> \
> if ((size) > (n)*CLIB_CACHE_LINE_BYTES)
> \
> __builtin_prefetch (_addr + (n)*CLIB_CACHE_LINE_BYTES, \
> CLIB_PREFETCH_##type,
> \
> /* locality */ 3);
>
> #define CLIB_PREFETCH(addr,size,type) \
> do {
> \
> void * _addr = (addr); \
>
> \
> ASSERT ((size) <= 4*CLIB_CACHE_LINE_BYTES); \
> _CLIB_PREFETCH (0, size, type); \
> _CLIB_PREFETCH (1, size, type); \
> _CLIB_PREFETCH (2, size, type); \
> _CLIB_PREFETCH (3, size, type); \
> } while (0)
> ‘’’
>
> [4]
> 4.1 CLIB_PREFETCH (p2->pre_data, 32, STORE);
> 4.2 CLIB_PREFETCH (p2->pre_data, 68, STORE);
> 4.3 CLIB_PREFETCH (b[4]->data, sizeof (ip0[0]), LOAD);
> 4.4 CLIB_PREFETCH (p2->data, 2*CLIB_CACHE_LINE_BYTES, LOAD);
>
> Thanks.
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