On Sat, Jul 23, 2022 at 12:04:33PM +0200, Christophe JAILLET wrote: > On one side, when using devm_kmalloc(), a memory overhead is added in order > to keep track of the data needed to release the resources automagically. > > On the other side, kmalloc() also rounds-up the required memory size in > order to ease memory reuse and avoid memory fragmentation. > > Both behavior together can lead to some over memory allocation which can > be avoided. > > For example: > - if 4096 bytes of managed memory is required > - "4096 + sizeof(struct devres_node)" bytes are required to the memory > allocator > - 8192 bytes are allocated and nearly half of it is wasted > > In such a case, it would be better to really allocate 4096 bytes of memory > and record an "action" to perform the kfree() when needed. > > On my 64 bits system: > sizeof(struct devres_node) = 40 > sizeof(struct action_devres) = 16 > > So, a devm_add_action() call will allocate 56, rounded up to 64 bytes. > > kmalloc() uses hunks of 8k, 4k, 2k, 1k, 512, 256, 192, 128, 96, 64, 32, 16, > 8 bytes. > > So in order to save some memory, if the 256 bytes boundary is crossed > because of the overhead of devm_kmalloc(), 2 distinct memory allocations > make sense. > > Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx> > --- > This patch is only a RFC to get feed-back on the proposed approach. > > It is compile tested only. > I don't have numbers to see how much memory could be saved. > I don't have numbers on the performance impact. > > Should this makes sense to anyone, I would really appreciate getting some > numbers from others to confirm if it make sense or not. > > > The idea of this patch came to me because of a discussion initiated by > Feng Tang <feng.tang@xxxxxxxxx>. He proposes to track wasted memory > allocation in order to give hints on where optimizations can be done. > > My approach is to avoid part of these allocations when due to the usage of > a devm_ function. > > > The drawbacks I see are: > - code is more complex > - this concurs to memory fragmentation because there will be 2 memory > allocations, instead of just 1 > - this is slower for every memory allocation because of the while loop > and tests > - the magic 256 constant is maybe not relevant on all systems > - some places of the kernel already take advantage of this over memory > allocation. So unpredictable impacts can occur somewhere! (see [1], > which is part of the [2] thread) > - this makes some assumption in devres.c on how memory allocation works, > which is not a great idea :( > > The advantages I see: > - in some cases, it saves some memory :) > - fragmentation is not necessarily an issue, devm_ allocated memory > are rarely freed, right? I think devm_ allocated memory does not happen that much, try it on your systems and see! Numbers would be great to have, can you run some benchmarks? Try it on a "common" SoC device (raspberry pi?) and a desktop to compare. thanks, greg k-h
