Subject: Re: [PATCH 0/5] Retry if fdopen() fails due to ENOMEM

From: Michael Haggerty <[email-masked]>
Date: Tue, 10 Mar 2015 12:42:56 +0100

On 03/05/2015 08:19 PM, Junio C Hamano wrote:
> Michael Haggerty <[email-masked]> writes:
> 
>> One likely reason for fdopen() to fail is the lack of memory for
>> allocating a FILE structure. When that happens, try freeing some
>> memory and calling fdopen() again in the hope that it will work the
>> second time.
> 
> In codepaths where we are likely under memory pressure, the above
> might help, but I have to wonder
> 
>     (1) if update-server-info and daemon fall into that category; and
> 
>     (2) if Git continues to work under such a memory pressure to
>         cause even fdopen() to fail.
> 
> In other words, I do not see a reason not to do this change, but I
> am not sure how much it would help us in practice.
> 
> We call fopen() from a lot more places than we call fdopen().  Do we
> want to do the same, or is there a good reason why this does not
> matter to callers of fopen(), and if so why doesn't the same reason
> apply to callers of fdopen()?

To be honest, I was hoping that Jonathan would jump in and respond to
this thread, as he is the one who suggested this change.

I agree that it seems rather unlikely that a call to fdopen() is the
straw that breaks the camel's back. But I've never looked very closely
at Git's facility for freeing up memory when an allocation fails and
don't really have a mental model for how it is used.

If memory is allocated profligately under the expectation that it can be
freed if necessary (i.e., if calls to try_to_free_routine() are
relatively routine and tend to free a lot of memory), then it seems
important that as many memory allocation paths as possible call it and
retry when there is an error. This would be the garbage-collection
model, in which a failure to call the garbage collector at the right
time unnecessarily turns a routine event into a fatal error.

On the other hand, if calling try_to_free_routine() is an act of
desperation and typically results in little or no memory being freed,
then calling it in this code path is probably not very useful.

You also make a good point that if this rigmarole makes sense for
fdopen(), then it probably also makes sense for fopen().

Michael

-- 
Michael Haggerty
[email-masked]