Hello!

On Fri, Dec 08, 2017 at 06:43:15PM +0000, debayang.qdt wrote:

> Hello,
>
> 64 bytes should be the minimum size.
>> Any links to support this claim?

AFAIK several aarch64 server vendors - centriq/thunderx/xgene/A53/A57.. implementations - uses either 64B/128B cache lines .
So keeping it at 64 bytes may be ok.
Also in linux kernel : https://elixir.free-electrons.com/linux/v4.14.4/source/arch/arm64/include/asm/cache.h#L73

> However, A53 is a small segment of arm64 based servers .
> Multiple aarch64 based servers uses 128 byte cache lines.
> For e.g Centriq 2400 aarch64 server from Qualcomm Datacenter Technologies uses 128 bytes cache line.
AFAIK, using a smaller cache line size is mostly fine and imply almost no performance difference in nginx. On ther other hand, using larger cache line size can easily result in multiple memory access where just one should be enough, resulting in suboptimal performance. That's why we generally assume 32 unless we know better.

> To be very accurate , we may read the dcache line size from CTR_EL0 register.
> Let me know what you think.

>> Given the level of popularity of aarch64 servers, I would rather not. I would prefer to rely on something architecture-independent like sysconf(_SC_LEVEL1_DCACHE_LINESIZE), though it looks like it doesn't work on aarch64 yet, see https://bugzilla.redhat.com/show_bug.cgi?id=1190638.

The aarch64 support is recently added in the glibc trunk .
https://sourceware.org/git/?p=glibc.git;a=commit;h=6d58ce5e5072945d44f2dba83ad16cd6febd056c
The implementation above reads the CTR_EL0 to get the cache information .

So , as you mentioned - it may make sense to use the sysconf to determine the cache line at runtime . In case it's not available on the system (value would be 0), we can fall back to the compile time defaults .
Below patch in nginx for the same.


# HG changeset patch
# User Debayan Ghosh <debayang.qdt@qualcommdatacenter.com>
# Date 1512811864 0
# Sat Dec 09 09:31:04 2017 +0000
# Node ID 217d0ed8730190a986c079c300169e10b90b4a82
# Parent c71aed1f6f88b8c5d56a4360ca4521a62a4c17ce
Nginx: Use sysconf to determine cacheline size

Using sysconf(_SC_LEVEL1_DCACHE_LINESIZE) to determine cache line size on the system.
This will make it architecture independent.
Falls back to using arch specific compile time defaults if above not supported.

diff -r c71aed1f6f88 -r 217d0ed87301 auto/unix
--- a/auto/unix Tue Nov 28 15:19:59 2017 +0000
+++ b/auto/unix Sat Dec 09 09:31:04 2017 +0000
@@ -963,6 +963,15 @@
ngx_feature_test="sysconf(_SC_NPROCESSORS_ONLN)"
. auto/feature

+ngx_feature="sysconf(_SC_LEVEL1_DCACHE_LINESIZE)"
+ngx_feature_name="NGX_HAVE_LEVEL1_DCACHE_LINESIZE"
+ngx_feature_run=no
+ngx_feature_incs=
+ngx_feature_path=
+ngx_feature_libs=
+ngx_feature_test="sysconf(_SC_LEVEL1_DCACHE_LINESIZE)"
+. auto/feature
+

ngx_feature="openat(), fstatat()"
ngx_feature_name="NGX_HAVE_OPENAT"
diff -r c71aed1f6f88 -r 217d0ed87301 src/os/unix/ngx_posix_init.c
--- a/src/os/unix/ngx_posix_init.c Tue Nov 28 15:19:59 2017 +0000
+++ b/src/os/unix/ngx_posix_init.c Sat Dec 09 09:31:04 2017 +0000
@@ -48,8 +48,17 @@
}

ngx_pagesize = getpagesize();
+#if (NGX_HAVE_LEVEL1_DCACHE_LINESIZE)
+ ngx_int_t cpu_cache_line = sysconf(_SC_LEVEL1_DCACHE_LINESIZE);
+ /* Below workaround to use default values on some environments
+ where the cache line size yet not available */
+ ngx_cacheline_size = (cpu_cache_line == 0) ?
+ NGX_CPU_CACHE_LINE : cpu_cache_line;
+#else
+ /* Use default cache line for the architecture */
ngx_cacheline_size = NGX_CPU_CACHE_LINE;
-
+#endif
+
for (n = ngx_pagesize; n >>= 1; ngx_pagesize_shift++) { /* void */ }

#if (NGX_HAVE_SC_NPROCESSORS_ONLN)



--Debayan
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