increased translation cache footprint in v2.6

Marcelo Tosatti
2005-06-26T22:59:20Z

Hi,

I'm sending this info because others also seem to be interested (and I assume there
are other small address translation cache machines where the following is is an issue).

We've noticed a slowdown while moving from v2.4 to v2.6 on a small PPC platform
(855T CPU running at 48Mhz, containing pair of separate I/D TLB caches with
32 entries each), with a relatively recent kernel (v2.6.11).

Test in question is a "dd" copying 16MB from /dev/zero to RAMDISK.

Pinning an 8Mbyte TLB entry at KERNELBASE brought performance back to v2.4 levels.

The following "itlb-content-before.txt" and "itlb-content-after.txt" files are the
contents of the virt. addresses cached in the I/TLB before and after a "sys_read"
system call.

[marcelo@logos itlb]$ diff -u 24-itlb-content-before.txt 24-itlb-content-after.txt | grep SPR | grep 816 | grep "+"
+SPR 816 : 0x0ffe800f 268337167
+SPR 816 : 0x0ffeb00f 268349455
+SPR 816 : 0xc009e01f -1073094625
+SPR 816 : 0xc009d01f -1073098721
+SPR 816 : 0xc000301f -1073729505
+SPR 816 : 0xc009c01f -1073102817

[marcelo@logos itlb]$ diff -u 24-itlb-content-before.txt 24-itlb-content-after.txt | grep SPR | grep 818 | grep "+" | wc -l
6

[marcelo@logos itlb]$ diff -u 26-itlb-before.txt 26-itlb-after.txt | grep 816 | grep SPR | grep "+"
+SPR 816 : 0x0feda16f 267231599
+SPR 816 : 0xc004b17f -1073434241
+SPR 816 : 0xc004a17f -1073438337
+SPR 816 : 0x0ff7e16f 267903343
+SPR 816 : 0x1001016f 268501359
+SPR 816 : 0xc000217f -1073733249
+SPR 816 : 0xc001617f -1073651329
+SPR 816 : 0xc002e17f -1073553025
+SPR 816 : 0xc010e17f -1072635521
+SPR 816 : 0xc002d17f -1073557121
+SPR 816 : 0xc010d17f -1072639617
+SPR 816 : 0xc000c17f -1073692289
+SPR 816 : 0xc000317f -1073729153

[marcelo@logos itlb]$ diff -u 26-itlb-before.txt 26-itlb-after.txt | grep 816 | grep SPR | grep "+" | wc -l
13

As can be seen the number of entries is more than twice (dominated by kernel addresses).

Sorry, I've got no list of functions for these addresses, but it was pretty obvious at the time
looking at the sys_read() codepath and respective virtual addresses.

Manual reorganization of the functions sounded too messy, although BenL mentions something about
fget_light() can and should be optimized.

I suppose the compiler/linker should be doing optimization based function order knowledge about
the on hottest paths (eg sys_read/sys_write). Why is it not doing this already and what are
the implications?

Date: Fri, 22 Apr 2005 12:39:21 -0300
Subject: v2.6 performance slowdown on MPC8xx: Measuring TLB cache misses

Here goes more data about the v2.6 performance slowdown on MPC8xx.

Thanks Benjamin for the TLB miss counter idea!

This are results of the following test script which zeroes the TLB counters,
copies 16MB of data from memory to memory using "dd", and reads the counters
again.

--

#!/bin/bash
echo 0 > /proc/tlbmiss
time dd if=/dev/zero of=file bs=4k count=3840
cat /proc/tlbmiss

--

The results:

v2.6: v2.4: delta
[root@CAS root]# sh script [root@CAS root]# sh script
real 0m4.241s real 0m3.440s
user 0m0.140s user 0m0.090s
sys 0m3.820s sys 0m3.330s

I-TLB userspace misses: 142369 I-TLB userspace misses: 2179 ITLB u: 139190
I-TLB kernel misses: 118288 I-TLB kernel misses: 1369 ITLB k: 116319
D-TLB userspace misses: 222916 D-TLB userspace misses: 180249 DTLB u: 38667
D-TLB kernel misses: 207773 D-TLB kernel misses: 167236 DTLB k: 38273

The sum of all TLB miss counter delta's between v2.4 and v2.6 is:

139190 + 116319 + 38667 + 38273 = 332449

Multiplied by 23 cycles, which is the average wait time to read a
page translation miss from memory:

332449 * 23 = 7646327 cycles.

Which is about 16% of 48000000, the total number of cycles this CPU
performs on one second. Its very likely that there is a significant
indirect effect of this TLB miss increase, other than the wasted
cycles to bring the page tables from memory: exception execution time
and context switching.

Checking "time" output, we can see 1s of slowdown:

[root@CAS root]# time dd if=/dev/zero of=file bs=4k count=3840

v2.4: v2.6: diff
real 0m3.366s real 0m4.360s 0.994s
user 0m0.080s user 0m0.111s 0.31s
sys 0m3.260s sys 0m4.218s 0.958s

Mostly caused by increased kernel execution time.

This proves that the slowdown is, in great part, due to increased
translation cache trashing.

Now, what is the best way to bring the performance back to v2.4 levels?

For this "dd" test, which is dominated by "sys_read/sys_write", I thought
of trying to bring the hotpath functions into the same pages, thus
decreasing the number of page translations required for such tasks.

Comments are appreciated.