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Re: [Qemu-devel] [PATCH for-1.2 v3 2/3] json-parser: don't replicate tok
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH for-1.2 v3 2/3] json-parser: don't replicate tokens at each level of recursion |
Date: |
Wed, 15 Aug 2012 14:04:52 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120717 Thunderbird/14.0 |
On 08/15/2012 12:45 PM, Michael Roth wrote:
> Currently, when parsing a stream of tokens we make a copy of the token
> list at the beginning of each level of recursion so that we do not
> modify the original list in cases where we need to fall back to an
> earlier state.
>
> In the worst case, we will only read 1 or 2 tokens off the list before
> recursing again, which means an upper bound of roughly N^2 token allocations.
>
> For a "reasonably" sized QMP request (in this a QMP representation of
> cirrus_vga's device state, generated via QIDL, being passed in via
> qom-set), this caused my 16GB's of memory to be exhausted before any
> noticeable progress was made by the parser.
>
> This patch works around the issue by using single copy of the token list
> in the form of an indexable array so that we can save/restore state by
> manipulating indices.
>
> A subsequent commit adds a "large_dict" test case which exhibits the
> same behavior as above. With this patch applied the test case successfully
> completes in under a second.
>
> Tested with valgrind, make check, and QMP.
>
> Signed-off-by: Michael Roth <address@hidden>
> ---
> json-parser.c | 230
> +++++++++++++++++++++++++++++++++++----------------------
> 1 file changed, 142 insertions(+), 88 deletions(-)
I'm not the most familiar with this code, so take my review with a grain
of salt, but I read through it and the transformation looks sane (and my
non-code findings from v2 were fixed).
Reviewed-by: Eric Blake <address@hidden>
> +static JSONParserContext parser_context_save(JSONParserContext *ctxt)
> +{
> + JSONParserContext saved_ctxt = {0};
> + saved_ctxt.tokens.pos = ctxt->tokens.pos;
> + saved_ctxt.tokens.count = ctxt->tokens.count;
> + saved_ctxt.tokens.buf = ctxt->tokens.buf;
Is it any simpler to condense 3 lines to 1:
saved_cts.tokens = ctxt->tokens;
> + return saved_ctxt;
> +}
> +
> +static void parser_context_restore(JSONParserContext *ctxt,
> + JSONParserContext saved_ctxt)
> +{
> + ctxt->tokens.pos = saved_ctxt.tokens.pos;
> + ctxt->tokens.count = saved_ctxt.tokens.count;
> + ctxt->tokens.buf = saved_ctxt.tokens.buf;
and again, ctxt->tokens = saved_ctxt.tokens;
--
Eric Blake address@hidden +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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