Editing On-demand accounting
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{{UBC toc}} | {{UBC toc}} | ||
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This page describes a very promising way of beancounters optimization. | This page describes a very promising way of beancounters optimization. | ||
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== Current accounting model == | == Current accounting model == | ||
Basically allocation of any kind of resource looks like this: | Basically allocation of any kind of resource looks like this: | ||
| − | < | + | <pre> |
struct some_resource *get_the_resource(int amount) | struct some_resource *get_the_resource(int amount) | ||
{ | { | ||
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return ret; | return ret; | ||
} | } | ||
| − | </ | + | </pre> |
We change this behaviour to work like this: | We change this behaviour to work like this: | ||
| − | < | + | <pre> |
struct some_resource *get_the_resource(int amount) | struct some_resource *get_the_resource(int amount) | ||
{ | { | ||
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return NULL; | return NULL; | ||
} | } | ||
| − | </ | + | </pre> |
| − | The <code>charge_beancounter()</code> call is responsible for checking whether the user is allowed to | + | The <code>charge_beancounter()</code> call is responsible for checking whether the user is allowed to consume desired 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 | + | Obviously, this change slows down the original code, as charge_beancounter() takes some slow operations like taking locks. We have an idea of how to optimize this behavior. |
== On-demand accounting basics == | == On-demand accounting basics == | ||
The main idea sonds like this: | The main idea sonds like this: | ||
| − | {{out|If the | + | {{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 | + | 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 == | == Example == | ||
Let's look at example of how this will work with the user memory accounting. | Let's look at example of how this will work with the user memory accounting. | ||
| − | Currently we account for the [[physpages]] resource | + | Currently we account for the [[physpages]] resource. That is -- the number of physical pages consumed by the processes. The accounting hooks are placed inside the page faults and hurt the performance. The accounting looks like this: |
| − | < | + | <pre> |
struct page *get_new_page(struct mm_struct *mm) | struct page *get_new_page(struct mm_struct *mm) | ||
{ | { | ||
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return NULL; | return NULL; | ||
} | } | ||
| − | </ | + | </pre> |
| − | However, we have a good | + | However, we have a good estimation of the RSS size -- that is the lenghts of mappings of the processes. Since the physical pages can only be allocated within these mappgins the RSS value can never exceed the sum of theis lenghs. The accounting will then look like this: |
| − | < | + | <pre> |
struct vm_area_struct *get_new_mapping(struct mm_struct *mm, | struct vm_area_struct *get_new_mapping(struct mm_struct *mm, | ||
unsigned long pages) | unsigned long pages) | ||
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return NULL; | return NULL; | ||
} | } | ||
| − | </ | + | </pre> |
| − | 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 | + | 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 increase the performance. |
Note, that the <code>recalculate_the_rss()</code> is called to calculate the exact RSS value on the beancounter. | Note, that the <code>recalculate_the_rss()</code> is called to calculate the exact RSS value on the beancounter. | ||