Details
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Technical task
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Resolution: Fixed
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Critical
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6.5.0
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None
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KV-Engine Mad-Hatter GA
Description
platform.so has an implementation of the 64bit byteswap functions ntohll() and htonll(), for platforms which don't have that symbol natively.
Profiling of ep-engine Writer threads highlighed that a large amount of time (~5%) was being spent in platform's ntohll() / htonll() functions. This was surprising, as:
- I had assumed that modern Linux (CentOS 7) provided the 64bit byteswap functions, and
- Even if the OS doesn't have those functions, I assumed our implementation shouldn't be that slow.
(For context the top 10 functions in the profile are below, ntohll is the 3rd hottest):
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Overhead Command Shared Object Symbol
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4.82% mc:writer_2 libsnappy.so.1.2.0 [.] snappy::internal::CompressFragment
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4.33% mc:writer_2 [kernel.kallsyms] [k] _raw_spin_lock_irq
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4.30% mc:writer_2 libplatform_so.so.0.1.0 [.] ntohll
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2.82% mc:writer_2 libc-2.17.so [.] __memcpy_ssse3
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2.49% mc:writer_2 libsnappy.so.1.2.0 [.] snappy::RawUncompress
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2.36% mc:writer_2 [kernel.kallsyms] [k] _raw_spin_lock
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1.99% mc:writer_2 libjemalloc.so.2 [.] je_malloc_usable_size
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1.74% mc:writer_2 [kernel.kallsyms] [k] __radix_tree_lookup
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1.43% mc:writer_2 libjemalloc.so.2 [.] je_malloc
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Both my assumptions are actually incorrect:
- CentOS 7 (and other recent distros including Ubuntu 18.04) don't have ntohll / htonll symbols. They do have functionally equivilent function htobe64() since glibc 2.9 (2008), but that's a different, Linux-specific symbol.
- Our implemenation is slow - it's doing old-style manual byteswap, which is 10x slower on mancouch:
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Run on (24 X 2400 MHz CPU s)
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2019-11-06 12:20:30
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-----------------------------------------------------
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Benchmark Time CPU Iterations
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-----------------------------------------------------
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Swap64 57 ns 57 ns 12216131
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BuiltinSwap64 5 ns 5 ns 141279127
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Given we already have an optimized byteswap implementation available from Folly, use that instead. Also inline the functions to reduce the call overhead.
Attachments
| For Gerrit Dashboard: MB-36776 | ||||||
|---|---|---|---|---|---|---|
| # | Subject | Branch | Project | Status | CR | V |
| 117439,4 | MB-36776: Replace inefficient impl of htonll/ntohll with folly::Endian | mad-hatter | platform | Status: MERGED | +2 | +1 |
| 117522,1 | Merge remote-tracking branch 'couchbase/mad-hatter' | master | platform | Status: MERGED | +2 | +1 |