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denc: do not always copy before decoding

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before this change, if the caller calls into ::decode_nohead(),
we will try to copy the memory chunk to be decoded before decoding it,
but if the buffer in the buffer list is not the last one, we will *deep*
copy the remaining part in the buffer list into a new contiguous memory
chunk and decode it instead. if we are decoding a map<int, buffer::ptr>
with lots of items in it, and the buffer::ptrs in it  are very small,
we will end up suffering from a serious internal fragmentation.

we could use the same strategy of decode(), where we compare the
size of remaining part with CEPH_PAGE_SIZE, and only copy it if
it's small enough. this requires that the decoded type supports
both variant of decoding contiguous and non-contiguous.
quite a few MDS types do not support both of them. for instance,
snapid_t only supports contiguous decoding.

so, instead of conditioning on size, in this change, we condition
on the traits::need_contiguous. probably we can condition on both
of them in a follow-up change.

Signed-off-by: Kefu Chai <kchai@redhat.com>
This commit is contained in:
Kefu Chai 2018-11-27 00:00:43 +08:00
parent c4544d1514
commit 719758a0d4
1 changed files with 10 additions and 6 deletions
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16
src/include/denc.h
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@ -1585,12 +1585,16 @@ inline std::enable_if_t<traits::supported && !traits::featured> decode_nohead(
return;
if (p.end())
throw buffer::end_of_buffer();
bufferptr tmp;
auto t = p;
t.copy_shallow(p.get_bl().length() - p.get_off(), tmp);
auto cp = std::cbegin(tmp);
traits::decode_nohead(num, o, cp);
p.advance(cp.get_offset());
if constexpr (traits::need_contiguous) {
bufferptr tmp;
auto t = p;
t.copy_shallow(p.get_bl().length() - p.get_off(), tmp);
auto cp = std::cbegin(tmp);
traits::decode_nohead(num, o, cp);
p.advance(cp.get_offset());
} else {
traits::decode_nohead(num, o, p);
}
}
}