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Subject[PATCH] memcpy_flushcache: use cache flusing for larger lengths
[ resending this to x86 maintainers ]

Hi

I tested performance of various methods how to write to optane-based
persistent memory, and found out that non-temporal stores achieve
throughput 1.3 GB/s. 8 cached stores immediatelly followed by clflushopt
or clwb achieve throughput 1.6 GB/s.

memcpy_flushcache uses non-temporal stores, I modified it to use cached
stores + clflushopt and it improved performance of the dm-writecache
target significantly:

dm-writecache throughput:
(dd if=/dev/zero of=/dev/mapper/wc bs=64k oflag=direct)
writecache block size 512 1024 2048 4096
movnti 496 MB/s 642 MB/s 725 MB/s 744 MB/s
clflushopt 373 MB/s 688 MB/s 1.1 GB/s 1.2 GB/s

For block size 512, movnti works better, for larger block sizes,
clflushopt is better.

I was also testing the novafs filesystem, it is not upstream, but it
benefitted from similar change in __memcpy_flushcache and
__copy_user_nocache:
write throughput on big files - movnti: 662 MB/s, clwb: 1323 MB/s
write throughput on small files - movnti: 621 MB/s, clwb: 1013 MB/s


I submit this patch for __memcpy_flushcache that improves dm-writecache
performance.

Other ideas - should we introduce memcpy_to_pmem instead of modifying
memcpy_flushcache and move this logic there? Or should I modify the
dm-writecache target directly to use clflushopt with no change to the
architecture-specific code?

Mikulas




From: Mikulas Patocka <mpatocka@redhat.com>

I tested dm-writecache performance on a machine with Optane nvdimm and it
turned out that for larger writes, cached stores + cache flushing perform
better than non-temporal stores. This is the throughput of dm-writecache
measured with this command:
dd if=/dev/zero of=/dev/mapper/wc bs=64 oflag=direct

block size 512 1024 2048 4096
movnti 496 MB/s 642 MB/s 725 MB/s 744 MB/s
clflushopt 373 MB/s 688 MB/s 1.1 GB/s 1.2 GB/s

We can see that for smaller block, movnti performs better, but for larger
blocks, clflushopt has better performance.

This patch changes the function __memcpy_flushcache accordingly, so that
with size >= 768 it performs cached stores and cache flushing. Note that
we must not use the new branch if the CPU doesn't have clflushopt - in
that case, the kernel would use inefficient "clflush" instruction that has
very bad performance.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
arch/x86/lib/usercopy_64.c | 36 ++++++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)

Index: linux-2.6/arch/x86/lib/usercopy_64.c
===================================================================
--- linux-2.6.orig/arch/x86/lib/usercopy_64.c 2020-03-24 15:15:36.644945091 -0400
+++ linux-2.6/arch/x86/lib/usercopy_64.c 2020-03-30 07:17:51.450290007 -0400
@@ -152,6 +152,42 @@ void __memcpy_flushcache(void *_dst, con
return;
}

+ if (static_cpu_has(X86_FEATURE_CLFLUSHOPT) && size >= 768 && likely(boot_cpu_data.x86_clflush_size == 64)) {
+ while (!IS_ALIGNED(dest, 64)) {
+ asm("movq (%0), %%r8\n"
+ "movnti %%r8, (%1)\n"
+ :: "r" (source), "r" (dest)
+ : "memory", "r8");
+ dest += 8;
+ source += 8;
+ size -= 8;
+ }
+ do {
+ asm("movq (%0), %%r8\n"
+ "movq 8(%0), %%r9\n"
+ "movq 16(%0), %%r10\n"
+ "movq 24(%0), %%r11\n"
+ "movq %%r8, (%1)\n"
+ "movq %%r9, 8(%1)\n"
+ "movq %%r10, 16(%1)\n"
+ "movq %%r11, 24(%1)\n"
+ "movq 32(%0), %%r8\n"
+ "movq 40(%0), %%r9\n"
+ "movq 48(%0), %%r10\n"
+ "movq 56(%0), %%r11\n"
+ "movq %%r8, 32(%1)\n"
+ "movq %%r9, 40(%1)\n"
+ "movq %%r10, 48(%1)\n"
+ "movq %%r11, 56(%1)\n"
+ :: "r" (source), "r" (dest)
+ : "memory", "r8", "r9", "r10", "r11");
+ clflushopt((void *)dest);
+ dest += 64;
+ source += 64;
+ size -= 64;
+ } while (size >= 64);
+ }
+
/* 4x8 movnti loop */
while (size >= 32) {
asm("movq (%0), %%r8\n"
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