Editing On-demand accounting

{{UBC toc}}
[[Category: Kernel internals]]

This page describes a very promising way of beancounters optimization.

== Current accounting model ==
Basically allocation of any kind of resource looks like this:
<source lang="c">
struct some_resource *get_the_resource(int amount)
{
        struct some_resource *ret;

        ret = find_or_allocate_the_resource(amount);
        return ret;
}
</source>
We change this behaviour to work like this:
<source lang="c">
struct some_resource *get_the_resource(int amount)
{
        struct some_resource *ret;

        if (charge_beancounter(amount) < 0)       
                return NULL;   

        ret = find_or_allocate_the_resource(amount);
        if (ret != NULL)
                return ret;

        uncharge_beancounter(amount);
        return NULL;
}
</source>
The <code>charge_beancounter()</code> call is responsible for checking whether the user is allowed to get the requested amount of the resource, i.e. if the resource consumption level is lower than the limit set.

Obviously, this change slows down the original code, as <code>charge_beancounter()</code> takes some slow operations like taking locks. We have an idea of how to optimize this behavior.

== On-demand accounting basics ==
The main idea sonds like this:
{{out|If the cunsumption level of any resource can be easily upper estimated with some value, and this setimation is lower than the limit, then we do not need to know the exact consumption level and allow the resource allocation without additional checks}}
Apparently, when the estimation exceeds the limit, we must switch to the slower mode, that will give us more presice value of the consumption level and (probably) allocate another portion of the resource.

== Example ==
Let's look at example of how this will work with the user memory accounting.

Currently we account for the [[physpages]] resource, that is, the number of physical pages consumed by a set of processes. The accounting hooks are placed inside the page fault handlers and thus hurting the performance. Currently accounting looks like this:
<source lang="c">
struct page *get_new_page(struct mm_struct *mm)
{
        struct page *pg;

        if (charge_beancounter(1) < 0)
                return NULL;

        pg = alloc_new_page(mm);
        if (pg != NULL)
                return pg;

        uncharge_beancounter(1);
        return NULL;
}
</source>

However, we have a good upper estimation of the RSS size — that is the lengths of mappings of the processes. Since the physical pages can only be allocated within these mappings, the RSS value can never exceed the sum of their lengths. The accounting will then look like this:
<source lang="c">
struct vm_area_struct *get_new_mapping(struct mm_struct *mm,
                unsigned long pages)
{
        if (!mm->fast_accounting)
                goto allocate;

        if (charge_beancounter(pages) == 0)
                goto allocate;

        mm->fast_accounting = 0;
        recalculate_the_rss(mm);

allocate:
        expand_mapping(mm);
}

struct page *get_new_page(struct mm_struct *mm)
{
        if (mm->fast_accounting)
                goto fast_path;

        if (charge_beancounter(1) < 0)
                return NULL;

fast_path:
        pg = alloc_new_page(mm);
        if (pg != NULL)
                return pg;

        if (!mm->fast_accounting)
                uncharge_beancounter(1);
        return NULL;
}
</source>
We do not call the slow <code>charge_beancounter()</code> function in the page fault (<code>get_new_page()</code>). Instead we account for the upper estimation in <code>get_new_mapping()</code> call that happens rarely and thus do not affect the performance.

Note, that the <code>recalculate_the_rss()</code> is called to calculate the exact RSS value on the beancounter.

== More things to do ==
In this model we switch from the fast acounting to the slow one. However, if the upper estimation becomes lower than the limit again we can switch back to the fast model. However, these switches are not very fast themselves, and being too frequent can hurt the performance instead of improving. So this would require some further investigations.