Wednesday Mar 30, 2011

Solaris Express 11

Since I have a brief project break, I thought it is time to now update this blog with what I have been doing the last one-and-a-half years since my last blog entry.

It is exciting that after many years of development, Solaris Express 11 was released in November 2010. In my next few blogs, I will talk about some exciting new features in Solaris Express 11. Stay tuned...

Saturday Aug 08, 2009

An interesting exercise investigating copy performance

Recently I was involved in a customer escalation. Our partner was complaining of poor copy performance seen by their application compared to what they saw on a flavor of Linux. Since copying is a key functionality to performance of many device drivers and subsystems, we felt that it was important to investigate this. This blog article will discuss our approach and outlines an interesting example on how to measure and investigate performance. Also, to set the tone, this discussion is mainly catered to X86 systems.

Our first endeavor was to have a benchmark to measure copy performance. While there are many tools such as libMicro to accomplish this, these have functions much beyond copying. As a result, the results exhibit significant variance and may not illustrate copy functionality.

We wrote a device driver, copydd, which will simply emulate a user-to-kernel copy operation. Then our benchmark, a simple user program opens copydd, and writes a 512 MB buffer to it in specified data sizes. Copydd does a copy from the user memory space to a preallocated memory buffer (which was allocated and initialized using an ioctl() before the benchmark started). We wrote this program for both Solaris and Linux, and the code is available for download here. Using this device driver, we could focus on the copy operation without getting sidetracked by anything else.

Our very first version of the benchmark measured the latency of writing into copydd by timing the routine at the user level since we believed that an improvement in latency would directly translate to improving bandwidth. We timed the benchmark for three different cases:

(i)Memory alloced but not touched before the benchmark,

(ii)Memory alloced and page-faulted in, and

(iii)Memory alloced and cached-in.

We discovered that using large pages (page size=2M instead of 4K) helped case (i) by 20% but did not affect cases (ii) and (iii). This would be indicative of TLB misses occurring when the memory is faulted-in. Setting use_sse_pagezero to 0 (using mdb -kw) helped case (ii) by close to 50%. The above setting makes the kernel zero-out a page right when it is faulted-in. In the process of zeroing-in, the page gets loaded into the cache, as a result of which the benchmark runs much faster.

We suggested our partner to tune use_sse_pagezero to 0 and re-evaluate their application. However, they reported little to no benefit.

We then decided to move over to measure copy bandwidth in our benchmark rather than the latency. This led us investigate the different approaches Solaris uses for copying data. Since copying is such critical functionality, it is implemented in assembly code for performance reasons. The code specific to Intel X86_64 based systems is available here. There are two variants of copy available (i)Cache-allocating copy, in which memory is first cached-in during read and then copied, and (ii)Non-temporal copy, in which copying happens from one memory location to another without bringing the contents into cache.

The choice between the two depends on whether the data copied will be immediately used by the CPU. For example, lets take the case of Network I/O. In the transmit path, a socket write() call copies the data from the user buffer to kernel (using uiomove()). Thereafter the packet is usually driven in the receive context (driven by processing of TCP ACKs). The CPU driving the data via the network stack is the one where the receive interrupt lands on. This CPU maybe very different from the one doing the user-to-kernel copy. Thus, bringing the data into cache (during the uiomove()) may cause unnecessary cache pollution on the CPU that did the copy, without actually benefitting from bringing the contents of the cache. Thus in this case a non-temporal copy is better since it does not cause any unnecessary cache pollution.

On the other hand, in the receive codepath, a driver (working in interrupt context) often copies the DMAed data into the kernel so that the receive DMA buffer can be freed as soon as possible. Thereafter the packets received are processed through the network protocol stack and delivered to the socket. In this case, since the data will immediately be accessed by the same thread which is copying it, and a cache allocating copy is potentially beneficial. Therefore, copy operations performed by device drivers use bcopy() or ddi_copyin() which do cache-allocating copy.

We then did an analysis of the performance of copy bandwidth using non-temporal copy (uiomove()) and cache-allocating copy (ddi_copyin()). Using the copydd driver on a Sun X4270 Lynx server based on the Intel Nehalem architecture, we arrived at the set of curves shown below.

The above curve shows that while ddi_copyin() is a small win over uiomove() for small data sizes(<1024 bytes), uiomove() beats by a factor of 2x when it comes to copying 128k chunks. The result is very interesting because it shows the tradeof between bringing contents from memory to cache, vs copying from one memory segment to the other.

Finally, to conclude this rather long blog, the tradeof between non-temporal copy (uiomove()) and cache-allocating copy(ddi_copyin()) depends on:
(i) The possibility of the data being copied to be used by the same thread as the one which did the copy.
(ii) The size of the data segment being copied.

In Solaris, we have this just right, socket write() and related APIs use uiomove(), while device drivers use ddi_copyin(). We contacted our partner who had complained to us about the copy performance and asked them if they were calling the right API. They switched over from ddi_copyin() to uiomove() and got nearly 2x performance benefit!!!

Monday Apr 13, 2009

Examining Crypto Performance on Intel Nehalem based Sun Fire X4170

Today, Sun is releasing a vast array of servers and blades with Intel's new Xeon 5560 (Nehalem) processor. We have significantly improved the performance of crypto algorithms (as part of Solaris Cryptographic Framework (SCF)). While some of these changes have been covered in my previous blogs, I would like to summarize them here.

I must first commend Dan Anderson for doing an excellent job in incorporating a lot of hand-coded assembly into the SCF. These enhancements were available in OpenSolaris 2008.10. Since then we have made the following enhancements, which will be available in OpenSolaris 2009.06. You can also try the preview bits of 2009.06 at genunix.org.

1) CR 6799218: RSA using Solaris Kernel Crypto framework lagging behind OpenSSL. We made changes that made RSA decrypt operations 1.8 times faster. The details are documented here.

2) CR 6811474 and CR 6823192 make number of changes to big_mont_mul() and big_mul() routines which form the essence of montogomery multiplication. These changes improve RSA decrypt operations by 10%.

3) CR 6812615: 64-bit RC4 has poor performance on Intel Nehalem. We made changes to the RC4 encrypt routine which delivered an improvement of 25% on Intel Nehalem. These changes are documented here.

The performance of these and other crypto algorithms may be examined using the PKCS#11 compliant Sun Software Crypto plugin. Applications can be linked to the library, /usr/lib/amd64/libpkcs11.so. For benchmarking the performance of SCF, we patched OpenSSL 0.9.8j (patch available at Jan Pachanec's blog) to use pkcs#11. The OpenSSL speed benchmark gives us the following numbers on a Sun Fire X4270 pre-release system with 2-socket Intel(r) Xeon(r)CPU X5560@2.8 GHz processor HT-enabled:

Benchmark 1-thread 16-threads
RSA-1024 encrypt 24.9 K ops/s 199.2 K op/s
RSA-1024 decrypt 1760 ops/s 14048op/s
RC4 encrypt (8k message) 317 MBytes/s 2265 Mbytes/s
MD5 Hash (8k message) 531 MBytes/s 6085 MBytes/s
SHA-1 Hash (8k message) 356 MBytes/s 2545 Mbytes/s
AES-256 encryption (8k message) 136.9 MBytes/s 1212.6 Mbytes/s


Please note that these numbers are with Hyper-Threading (HT) enabled on the Nehalem processor, in which two virtual processors share the same execution pipeline. The performance of all algorithms is seen to scale pretty linearly from one-core to 8-cores. Disabling HT did not make much of a difference to the benchmarks, and this could be because crypto algorithmic operations do not have many stalls in the execution pipeline, and therefore the benefit from having virtual processors is less.

For further notes on Sun's Intel Nehalem based servers and blades, I recommend you to read Heather's blog which cross-links all Nehalem-based blog entries. And please do leave your comments and feedback behind.

Tuesday Apr 07, 2009

Benchmarking (and improving the benchmark) for RSA decrypt

I wrote about our work on RSA-decrypt in OpenSolaris two posts ago. One of the biggest obstacles we faced while improving RSA is that the bignum library (which RSA uses for the expensive multiplication routines) is a kernel module. Anything in kernel land is so much harder to play around with -- analyze, improve, debug, measure performance --, and so we worked on porting the kernel code to a userland program. We have something ready now, and the code in userland looks exactly as what is in the current version of bignum in Opensolaris.

More interestingly, we can use this code as a simple benchmark for CPU performance. The current code has been tuned for performance for only x86_64 (CMT has hardware accelerators; therefore software performance of crypto algorithms does not interest us much). As an example, here are the results for RSA1024 decrypt on various systems (and processors):

Sun Fire X4150 (Intel Xeon 5450 3.16 GHz Processor) 493544 nsec
Sun Fire X4600 (AMD Opteron(tm) Processor 885, 2.6 GHz) 557934 nsec
Sun Fire X4200 (AMD Opteron(tm) Processor 280, 2.31 GHz) 606147 nsec


The benchmark code is available for download here.

However, our main objective remains to improve this benchmark, have better performance on RSA decrypt, and then deliver it to OpenSolaris. This effort has already led to a CR (which will be fixed soon in OpenSolaris). Hopefully you can contribute more improvements to the code. Please send in your ideas by e-mail or post a comment to this blog.

Here is how the benchmark performs on a X4150 (2-socket quad-core Intel Xeon 3.16 GHz processor). The hottest function is big_mul_add_vec, which is already implemented in assembly for x64. The analysis is done using collect/er_print tools available in Sun Studio 12.

%collect ./bignum_test
Creating experiment database test.1.er ...

%er_print -functions test.1.er
Functions sorted by metric: Exclusive User CPU Time

Excl.     Incl.      Name  
User CPU  User CPU         
  sec.      sec.      
51.956    51.956     
26.899    26.899     big_mul_add_vec
 9.236    47.623     big_mont_mul
 4.953    11.428     big_sqr_vec
 3.663    18.863     big_mul
 1.601     1.601     big_mul_set_vec
 1.181    49.234     big_modexp_ncp_int
 0.921     0.921     big_sub_vec
 0.570     0.570     big_sub_pos
 0.380     0.380     rand_r
 0.370     0.911     genrandomstring
 0.280     3.613     big_mul_vec
 0.240     0.240     big_cmp_abs
 0.220     1.301     big_div_pos
 0.170     0.170     big_mulhalf_high
 0.160     0.160     _free_unlocked
 0.160     0.160     big_copy
 0.160     0.540     rand
 0.120     0.120     big_mulhalf_low
 0.120     0.140     mutex_unlock
 0.070     0.130     _malloc_unlocked
 0.070     0.640     big_sub_pos_high
 0.050     0.050     big_shiftleft
 0.050     0.050     sigon
 0.040     0.070     mutex_lock_impl
 0.030     0.030     _smalloc
 0.030     0.300     big_init1
 0.030     0.030     cleanfree
 0.030     0.030     gettimeofday
 0.020     0.090     big_cmp_abs_high
 0.020     0.310     big_finish
 0.020     0.020     big_n0
 0.020     0.290     free
 0.020     0.270     malloc
 0.020     0.090     mutex_lock
 0.010     0.010     big_add_abs
 0.010     0.010     big_numbits
 0.010     0.010     big_shiftright
 0.        0.        _init
 0.        0.        _rt_boot
 0.        0.        _setup
 0.       51.956     _start
 0.       51.016     big_modexp_crt_ext
 0.       50.065     big_modexp_ext
 0.        0.200     big_mont_conv
 0.        0.490     big_mont_rr
 0.        0.        call_init
 0.        0.        dtrace_dof_init
 0.        0.        ioctl
 0.       51.956     main
 0.        0.        setup
About

This blog discusses my work as a performance engineer at Sun Microsystems. It touches upon key topics of performance issues in operating systems and the Solaris Networking stack.

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