Re: [Qemu-devel] [PATCH 07/15] qapi: use mmap for QmpInputVisitor

[Eric Blake <eblake@redhat.com>]

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On 06/28/2016 05:36 AM, Paolo Bonzini wrote:
> 
> 
> On 28/06/2016 11:01, Peter Lieven wrote:
>> this struct is approx 75kB
>>
>> Signed-off-by: Peter Lieven <pl@kamp.de>
>> ---
>>  qapi/qmp-input-visitor.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> Can you change the stack to a QSLIST instead?  That's where most of the
> waste comes from.

QmpInputVisitor has:

struct QmpInputVisitor
{
    Visitor visitor;

    /* Root of visit at visitor creation. */
    QObject *root;

    /* Stack of objects being visited (all entries will be either
     * QDict or QList). */
    StackObject stack[QIV_STACK_SIZE];
...

while QmpOutputVisitor has:

struct QmpOutputVisitor
{
    Visitor visitor;
    QStack stack; /* Stack of containers that haven't yet been finished */
    QObject *root; /* Root of the output visit */
    QObject **result; /* User's storage location for result */
};

The extra layer of indirection to a QStack vs. a direct array has
tradeoffs, but both Markus and I have commented in the past that both
files' stacks are rather wasteful, and we just have not had a reason to
improve them.  This thread may be the reason :)

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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