On 04/01/2011 03:00 AM, Tom Hughes wrote:
> On 01/04/11 10:50, Julian Seward wrote:
>>
>> [... mucho discussion ...]
>>
>> I'm now comprehensively confused. Is there any bug/failure in
>> Memcheck resulting from this, that needs to be looked at?
>
> There is no clear evidence of one at the moment.
>
> The current situation is that we have proved that the test program
> really did have a leak that was correctly reported. The only outstanding
> question there was about why memory allocation was failing under
> valgrind when it worked normally but that is expected.
There is a failure, at least from the user's perspective. When run under
memcheck, then the [corrected] test program runs out of space much sooner
than expected. The space overhead appears to be greater than 20 to 1,
when something like 3:1 might be "normal". When the size of individual
blocks is huge (exceeds (RAM + SWAP)/400, for example) then the default
settings tend to exhaust space very quickly, with symptoms that are
mysterious to non-experts. This is a Usability bug.
Bart Van Assche suggests --freelist-vol=0, which is an excellent suggestion.
For better usability, then memcheck should take more care with huge blocks.
Put a bound of (RAM + SWAP)/8 on the volume of the freelist; or, keep a
separate freelist for huge blocks, and age them faster. For a growing
realloc() of a huge block, then relax the strategy which forces malloc_new +
copy + free_old. Give up some local error detection for the chance to
live long enough to detect more errors later.
If nothing else, then when the first huge block is detected give a warning:
huge block detected (%lu bytes); consider adjusting --freelist-vol
[The values of MemTotal and SwapTotal are in /proc/meminfo. Evaluating
the threshold once is enough even though /sbin/swapon or pluggable RAM
might change it. Disregarding other processes is OK because even novices
know to minimize external contention.]
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